diff --git a/CHANGELOG.md b/CHANGELOG.md
index d36550d8..321bbf4e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,7 @@
 
 ### Fixed
 
+- Add a missing separator when appending to an array whose last value is followed by standalone comments.
 - Arithmetic and `replace()` results for `Integer`, `Float`, `String`, `DateTime`, `Date` and `Time` no longer share the original item's `Trivia`, so changing the result's comment or indentation no longer rewrites the item still stored in the document.
 
 ## [0.15.2] - 2026-10-16
diff --git a/tests/test_items.py b/tests/test_items.py
index 36f4afd2..ee6174af 100644
--- a/tests/test_items.py
+++ b/tests/test_items.py
@@ -521,6 +521,28 @@ def test_modify_array_with_comment() -> None:
     assert doc.as_string() == "x = [\n    2\n]"
 
 
+@pytest.mark.parametrize("trailing_comma", [False, True])
+@pytest.mark.parametrize("operation", ["append", "extend", "insert"])
+@pytest.mark.parametrize("comments", ["# trailing\n", "# first\n# trailing\n"])
+def test_append_after_standalone_array_comments(
+    trailing_comma: bool, operation: str, comments: str
+) -> None:
+    doc = parse("x = [\n1" + ("," if trailing_comma else "") + "\n" + comments + "]\n")
+    array = doc["x"]
+    if operation == "append":
+        array.append(2)
+    elif operation == "extend":
+        array.extend([2, 3])
+    else:
+        array.insert(len(array) + 5, 2)
+
+    expected = [1, 2, 3] if operation == "extend" else [1, 2]
+    rendered = doc.as_string()
+    assert array == expected
+    assert parse(rendered)["x"] == expected
+    assert comments in rendered
+
+
 def test_append_to_multiline_array_with_comment() -> None:
     doc = parse(
         """\
diff --git a/tomlkit/items.py b/tomlkit/items.py
index a73fab8a..6cc0515f 100644
--- a/tomlkit/items.py
+++ b/tomlkit/items.py
@@ -1675,6 +1675,10 @@ def insert(self, pos: int, value: Any) -> None:  # type: ignore[override]
             indent = self._value[idx].indent
         if idx > 0:
             last_item = self._value[idx - 1]
+            last_value_item = last_item
+            if pos > 0 and length:
+                # Standalone comments may follow the preceding array value.
+                last_value_item = self._value[self._index_map[min(pos, length) - 1]]
             if indent is None:
                 indent = last_item.indent
             if not isinstance(last_item.value, Null) and "\n" in default_indent:
@@ -1683,14 +1687,14 @@ def insert(self, pos: int, value: Any) -> None:  # type: ignore[override]
                 comma = last_item.comma
             comma_in_indent = indent is not None and "," in indent.s
             if (
-                last_item.comma is None
-                and not isinstance(last_item.value, Null)
+                last_value_item.comma is None
+                and not isinstance(last_value_item.value, Null)
                 and not comma_in_indent
             ):
                 # Add comma to the last item to separate it from the following
                 # items, unless the new item's indent already starts with a
                 # comma (comma-first style).
-                last_item.comma = Whitespace(",")
+                last_value_item.comma = Whitespace(",")
         if indent is not None and "," in indent.s:
             # Comma-first style: the item that will follow the new one brings
             # its own leading comma, so the new item must not add a trailing