diff --git a/api/src/database/UserDB.ts b/api/src/database/UserDB.ts
index 2cdec6cb..ae8d7e49 100644
--- a/api/src/database/UserDB.ts
+++ b/api/src/database/UserDB.ts
@@ -50,21 +50,6 @@ export class UserDB {
                 WHERE userID = $1`, [uuid])
             .then(extract).then(map(u => u.samlid)).then(one);
     }
-    static async getUserByPossibleMentionInCourse(possibleMention: string, courseID: string, params: DBTools = {}) {
-        const {client = pool} = params;
-        const courseid = UUIDHelper.toUUID(courseID);
-        return client.query(`
-        SELECT *
-        FROM "UsersView" as u, "CourseRegistration" as cr
-        WHERE
-            (u.userID = cr.userID)
-        AND (courseID = $1)
-        AND (POSITION(userName in $2) = 1)
-        ORDER BY (char_length(userName)) DESC
-        LIMIT 1
-        `, [courseid, possibleMention])
-            .then(extract).then(one).then(userToAPI);
-    }
 
     /**
      *
diff --git a/api/src/database/dbTester.ts b/api/src/database/dbTester.ts
index 206127b4..c99a3f7e 100644
--- a/api/src/database/dbTester.ts
+++ b/api/src/database/dbTester.ts
@@ -86,7 +86,6 @@ async function DBusersTest() {
     await promise(UH.getAllStudents(), "getAllStudents");
     await promise(UH.getUserByID(uuid0), "getUserById");
     await promise(UH.searchUser("CAS"), "searchUser");
-    await promise(UH.getUserByPossibleMentionInCourse("Caas rest of comment", uuid0), "UserMention");
     await promise(UH.filterUserInCourse({courseID: uuid1}), "searchuserInCourse");
     const u1 = await promise(UH.getUserBySamlID("samling_admin"), "samlgetter");
     const samlid = await promise(UH.getSamlIDForUserID(u1.ID), "saml by user");
diff --git a/api/src/helpers/MentionsHelper.ts b/api/src/helpers/MentionsHelper.ts
index 2bf49a63..1fa1253c 100644
--- a/api/src/helpers/MentionsHelper.ts
+++ b/api/src/helpers/MentionsHelper.ts
@@ -4,31 +4,16 @@ import {CourseRole} from "../../../models/enums/CourseRoleEnum";
 import {containsPermission, PermissionEnum} from "../../../models/enums/PermissionEnum";
 
 import {CourseRegistrationDB} from "../database/CourseRegistrationDB";
-import {NotFoundDatabaseError} from "../database/DatabaseErrors";
 import {pgDB} from "../database/HelperDB";
 import {MentionsDB} from "../database/MentionsDB";
-import {UserDB} from "../database/UserDB";
+import {MentionsUserCache} from "./MentionsUserCache";
 
 /** Get the parts of the a comment following the @-sign, which starts a mention */
 export function getPossibleMentions(commentBody: string) {
     return commentBody.split("@").slice(1).map(mention => mention.trim());
 }
 
-/**
- * Make a database call to find a user that matches the first part of a possible mention,
- * returning the possbileMention if none is found
- */
-async function getUserForPossibleMention(possibleMention: string, courseID: string, client?: pgDB) {
-    try {
-        return await UserDB.getUserByPossibleMentionInCourse(possibleMention, courseID, {client});
-    } catch (err) {
-        if (err instanceof NotFoundDatabaseError) {
-            return possibleMention;
-        } else {
-            throw err;
-        }
-    }
-}
+const usersCache = new MentionsUserCache();
 
 /** Find a course role that matches the first part of a possible mention */
 function getRoleForPossibleMention(possibleMention: string) {
@@ -44,7 +29,7 @@ function getRoleForPossibleMention(possibleMention: string) {
 export async function getMentions(commentBody: string, courseID: string, client?: pgDB) {
     const users = await Promise.all(
         getPossibleMentions(commentBody)
-            .map(async pm => getUserForPossibleMention(pm, courseID, client))
+            .map(async pm => usersCache.getUserForPossibleMention(pm, courseID, client))
     );
     return users
         .map(userOrComment =>
diff --git a/api/src/helpers/MentionsUserCache.ts b/api/src/helpers/MentionsUserCache.ts
new file mode 100644
index 00000000..a0d04d50
--- /dev/null
+++ b/api/src/helpers/MentionsUserCache.ts
@@ -0,0 +1,47 @@
+import {pgDB} from "../database/HelperDB";
+import {User} from "../../../models/api/User";
+import {PrefixLookupTree} from "./PrefixLookupTree";
+import {UserDB} from "../database/UserDB";
+
+interface CacheItem {
+    lookupTree: PrefixLookupTree<User>;
+    lastUpdated: Date;
+}
+
+/** Caching layer for mention lookups to spare the database. */
+export class MentionsUserCache {
+    /** Map to store the cache, indexed by courseID. */
+    private cache: Map<string, CacheItem> = new Map();
+    private expirationMs = 1000 * 60 * 60; // 1 hour
+
+    /** Get the prefix lookup tree associated with a certain course. */
+    private async getLookupTree(courseID: string, client?: pgDB) {
+        // Try to get the lookup tree from the cache.
+        const item = this.cache.get(courseID);
+        // If it is not in the cache, or it is expired,
+        if (item === undefined || item.lastUpdated.getTime() < Date.now() - this.expirationMs) {
+            // then get the full userlist for this course from the database,
+            const users = await UserDB.filterUserInCourse({courseID, client});
+            // create the lookup tree for it,
+            const lookupTree = PrefixLookupTree.ofList(users, u => u.name);
+            // store it in the cache,
+            this.cache.set(courseID, {lookupTree, lastUpdated: new Date()});
+            // and return it.
+            return lookupTree;
+        } else {
+            // If it is in the cache, and it is not expired, return it.
+            return item.lookupTree;
+        }
+    }
+
+    /** Get the user that is mentioned at the start of the possibleMention string. */
+    public async getUserForPossibleMention(possibleMention: string, courseID: string, client?: pgDB) {
+        const lookupTree = await this.getLookupTree(courseID, client);
+        const user = lookupTree.lookup(possibleMention);
+        if (user === undefined) {
+            return possibleMention;
+        } else {
+            return user;
+        }
+    }
+}
diff --git a/api/src/helpers/PrefixLookupTree.ts b/api/src/helpers/PrefixLookupTree.ts
new file mode 100644
index 00000000..cd70fe05
--- /dev/null
+++ b/api/src/helpers/PrefixLookupTree.ts
@@ -0,0 +1,110 @@
+/** A prefix lookup tree, used to e.g. find the user that is mentioned in a comment.
+ * The structure is as follows, e.g. for the user named Cas:
+ * { "C": { "a": { "s": { undefined: (user Cas) } } } }
+ * If we have three users, Cas, Caas and Cass, the structure looks like this:
+ * { "C": {
+ *     "a": {
+ *       "a": {
+ *         "s": { match: (user Caas) }
+ *       },
+ *       "s": {
+ *         match: (user Cas),
+ *         "s": { match: (user Cass) }
+ *       }
+ *   }
+ * }
+ * A lookup is performed by traversing the tree, for example with the comment "@Cas help me!"
+ * we try to look up the longest name that matches "Cas help me!": first we can match "C", which
+ * returns a tree with no match value (as there is no user named "C") and a next character "a".
+ * Our next character is "a", so we choose that next tree, which again has no match value and
+ * next characters "a" and "s". Our next character is "s", so we choose that tree. This tree has
+ * a match value (the user Cas), and a next character "s", but our next character is a space.
+ * Therefore we return the last match value we encountered while traversing the tree, which in
+ * this case is the match value at the last node we visited. In general, it could happen that we
+ * encounter more characters that have branches down the tree, but we never reach deep enough to
+ * reach another matching character. In this case the last match value will be farther up the tree.
+ */
+export class PrefixLookupTree<T> {
+    /** The default value to choose if there are no more matching characters. */
+    private match: T | undefined;
+    /** Map of next character to be matched and each corresponding subtree. */
+    private next: Map<string, PrefixLookupTree<T>>;
+
+    /** Create an empty tree. Use the ofList static method instead, if you want to create
+     * a tree from a list of values.
+     */
+    public constructor() {
+        this.match = undefined;
+        this.next = new Map();
+    }
+
+    /** Build a tree from a list of values.
+     * @param list The values to insert into the tree.
+     * @param getKey A function that returns the key for a value.
+     */
+    public static ofList<T>(list: T[], getKey: (value: T) => string): PrefixLookupTree<T> {
+        const tree = new PrefixLookupTree<T>();
+        for (const value of list) {
+            tree.insert(getKey(value), value);
+        }
+        return tree;
+    }
+
+    /** Insert an item into the tree.
+     * @param key The key to insert. Note that the key should be unique, existing items are overriden.
+     * @param value The value to insert.
+     */
+    public insert(key: string, value: T) {
+        // If the key is the empty string, then we have a match on this subtree.
+        if (key === "") {
+            // Thus we set the match value to the given value.
+            this.match = value;
+        } else {
+            // Else, we traverse the tree based on the next character of the text we want to match with.
+            let next = this.next.get(key[0]);
+            // If there is no subtree for the next character,
+            if (next === undefined) {
+                // we create one
+                next = new PrefixLookupTree<T>();
+                // and store it in the current map of subtrees.
+                this.next.set(key[0], next);
+            }
+            // Then we can recursively add the remainder of the key to this subtree.
+            next.insert(key.slice(1), value);
+        }
+    }
+
+    /** Find the longest prefix stored in the tree that matches the start of the given text. */
+    public lookup(text: string): T | undefined {
+        return this._lookup(text);
+    }
+
+    /** This is an internal version of lookup, which also takes the current longest match we have already found. */
+    private _lookup(text: string, longestMatch?: T): T | undefined {
+        // If we have no more characters to match, then this subtree is the longest possible match.
+        if (text === "") {
+            // If this subtree has a match value, then the text fully matches to this subtree
+            if (this.match !== undefined) {
+                // and thus we return the match value.
+                return this.match;
+            } else {
+                // Else, we return the longest match that we have encountered along the way.
+                return longestMatch;
+            }
+        } else {
+            // If there is a match at the current subtree, then that is the new longest match we have found.
+            const newLongest = this.match !== undefined ? this.match : longestMatch;
+            // If there are characters to match, we check if there is a subtree for the next character.
+            const next = this.next.get(text[0]);
+            if (next === undefined) {
+                // If there is no such subtree, then we cannot continue our match and so we return the longest
+                // match that we encountered up to this point.
+                return newLongest;
+            } else {
+                // If that subtree exists, we recursively call _lookup on that subtree with the remainder of the text,
+                // propagating the longest match we have found so far.
+                return next._lookup(text.slice(1), newLongest);
+            }
+        }
+    }
+}
diff --git a/test/api/helpers/PrefixLookupTree.spec.ts b/test/api/helpers/PrefixLookupTree.spec.ts
new file mode 100644
index 00000000..cd4b271a
--- /dev/null
+++ b/test/api/helpers/PrefixLookupTree.spec.ts
@@ -0,0 +1,77 @@
+import "mocha";
+import {expect} from "chai";
+
+import {PrefixLookupTree} from "../../../api/src/helpers/PrefixLookupTree";
+
+describe("PrefixLookupTree", () => {
+    it("should return exact match for a single value", () => {
+        const sut = new PrefixLookupTree();
+        sut.insert("test", "value");
+        expect(sut.lookup("test")).to.equal("value");
+    });
+
+    it("should return prefix for a single value", () => {
+        const sut = new PrefixLookupTree();
+        sut.insert("test", "value");
+        expect(sut.lookup("test string")).to.equal("value");
+    });
+
+    it("should return undefined if single value doesn't fully match", () => {
+        const sut = new PrefixLookupTree();
+        sut.insert("test", "value");
+        expect(sut.lookup("tes")).to.be.undefined;
+    });
+
+    it("should return the longest exact match of two", () => {
+        const sut = new PrefixLookupTree();
+        sut.insert("test", "value1");
+        sut.insert("test string", "value2");
+        expect(sut.lookup("test string")).to.equal("value2");
+    });
+
+    it("should return the longest prefix of two", () => {
+        const sut = new PrefixLookupTree();
+        sut.insert("test", "value1");
+        sut.insert("test string", "value2");
+        expect(sut.lookup("test string test")).to.equal("value2");
+    });
+
+    it("should backtrack if a possible longer match doesn't fully match", () => {
+        const sut = new PrefixLookupTree();
+        sut.insert("test", "value1");
+        sut.insert("test string", "value2");
+        expect(sut.lookup("test strong")).to.equal("value1");
+    });
+
+    it("should still return the longest prefix when backtracking", () => {
+        const sut = new PrefixLookupTree();
+        sut.insert("t", "value1");
+        sut.insert("test", "value2");
+        sut.insert("test string", "value3");
+        expect(sut.lookup("test strong")).to.equal("value2");
+    });
+
+    it("should return the longest prefix out of three fully distinct", () => {
+        const sut = new PrefixLookupTree();
+        sut.insert("one", "value1");
+        sut.insert("two", "value2");
+        sut.insert("four", "value4");
+        expect(sut.lookup("two and a half")).to.equal("value2");
+    });
+
+    it("should return the longest prefix out of several overlapping", () => {
+        const sut = new PrefixLookupTree();
+        sut.insert("abc", "value1");
+        sut.insert("abd", "value2");
+        sut.insert("abcd", "value3");
+        sut.insert("aa", "value4");
+        sut.insert("abcde", "value5");
+
+        expect(sut.lookup("a a")).to.be.undefined;
+        expect(sut.lookup("abc def")).to.equal("value1");
+        expect(sut.lookup("abdef")).to.equal("value2");
+        expect(sut.lookup("abcd e")).to.equal("value3");
+        expect(sut.lookup("aa b")).to.equal("value4");
+        expect(sut.lookup("abcde f")).to.equal("value5");
+    });
+});