diff --git a/openc3/lib/openc3/models/plugin_model.rb b/openc3/lib/openc3/models/plugin_model.rb
index ec816e88e0..5a5d4b45d8 100644
--- a/openc3/lib/openc3/models/plugin_model.rb
+++ b/openc3/lib/openc3/models/plugin_model.rb
@@ -598,14 +598,12 @@ def self.gem_names
     def self.resolve_pypi_url(scope:)
       pypi_url = nil
       begin
-        pypi_url = get_setting('pypi_url', scope: scope)
-        pypi_url += '/simple' if pypi_url
+        pypi_url = PypiUrl.index_url(get_setting('pypi_url', scope: scope))
       rescue => e
         Logger.error("Failed to retrieve pypi_url: #{e.formatted}")
       ensure
         if pypi_url.nil?
-          pypi_url = ENV.fetch('PYPI_URL', nil)
-          pypi_url += '/simple' if pypi_url
+          pypi_url = PypiUrl.index_url(ENV.fetch('PYPI_URL', nil))
           pypi_url ||= PypiUrl::DEFAULT
         end
       end
diff --git a/openc3/lib/openc3/models/python_package_model.rb b/openc3/lib/openc3/models/python_package_model.rb
index c7f96738f3..d91e055240 100644
--- a/openc3/lib/openc3/models/python_package_model.rb
+++ b/openc3/lib/openc3/models/python_package_model.rb
@@ -235,19 +235,13 @@ def self.install(name_or_path, scope:, plugin: nil)
       end
       package_filename = File.basename(package_file_path)
       begin
-        pypi_url = get_setting('pypi_url', scope: scope)
-        if pypi_url
-          pypi_url += '/simple'
-        end
+        pypi_url = PypiUrl.index_url(get_setting('pypi_url', scope: scope))
       rescue => e
         Logger.error("Failed to retrieve pypi_url: #{e.formatted}")
       ensure
         if pypi_url.nil?
           # If Redis isn't running try the ENV, then simply pypi.org/simple
-          pypi_url = ENV.fetch('PYPI_URL', nil)
-          if pypi_url
-            pypi_url += '/simple'
-          end
+          pypi_url = PypiUrl.index_url(ENV.fetch('PYPI_URL', nil))
           pypi_url ||= PypiUrl::DEFAULT
         end
       end
diff --git a/openc3/lib/openc3/utilities/pypi_url.rb b/openc3/lib/openc3/utilities/pypi_url.rb
index a9db1d38cc..cc951a42d9 100644
--- a/openc3/lib/openc3/utilities/pypi_url.rb
+++ b/openc3/lib/openc3/utilities/pypi_url.rb
@@ -18,6 +18,18 @@ module OpenC3
   class PypiUrl
     DEFAULT = 'https://pypi.org/simple'
 
+    # Turn a configured PyPI base URL (the pypi_url setting or PYPI_URL) into
+    # its simple index URL. Trailing slashes are dropped first, matching the
+    # ${PYPI_URL%/}/simple the image builds use, so https://mirror/ does not
+    # give https://mirror//simple and https://pypi.org/ still equals DEFAULT.
+    #
+    # @param base_url [String, nil] the configured base url
+    # @return [String, nil] the simple index url, or nil when base_url is nil
+    def self.index_url(base_url)
+      return nil if base_url.nil?
+      "#{base_url.sub(%r{/+\z}, '')}/simple"
+    end
+
     # Validate that a resolved pypi_url is an http(s) URL before it is handed to
     # pip. The value can come from a user-writable setting or ENV, so a malformed
     # or non-http value is rejected and replaced with the default rather than
diff --git a/openc3/spec/models/plugin_model_spec.rb b/openc3/spec/models/plugin_model_spec.rb
index a5eed5925f..779d397820 100644
--- a/openc3/spec/models/plugin_model_spec.rb
+++ b/openc3/spec/models/plugin_model_spec.rb
@@ -729,6 +729,24 @@ def install_plugin
         expect(PluginModel.resolve_pypi_url(scope: "DEFAULT")).to eql "https://custom.pypi.example.com/simple"
       end
 
+      it "drops a trailing slash from the pypi_url setting" do
+        allow(PluginModel).to receive(:get_setting).with('pypi_url', scope: "DEFAULT").and_return("https://custom.pypi.example.com/")
+        expect(PluginModel.resolve_pypi_url(scope: "DEFAULT")).to eql "https://custom.pypi.example.com/simple"
+      end
+
+      it "treats the public index with a trailing slash as the default" do
+        allow(PluginModel).to receive(:get_setting).with('pypi_url', scope: "DEFAULT").and_return("https://pypi.org/")
+        pypi_url = PluginModel.resolve_pypi_url(scope: "DEFAULT")
+        expect(pypi_url).to eql PypiUrl::DEFAULT
+        expect(PluginModel.build_pypi_args(pypi_url)).to_not include("--no-config")
+      end
+
+      it "drops a trailing slash from ENV PYPI_URL" do
+        allow(PluginModel).to receive(:get_setting).with('pypi_url', scope: "DEFAULT").and_return(nil)
+        allow(ENV).to receive(:fetch).with('PYPI_URL', nil).and_return("https://env.pypi.example.com/")
+        expect(PluginModel.resolve_pypi_url(scope: "DEFAULT")).to eql "https://env.pypi.example.com/simple"
+      end
+
       it "falls back to ENV PYPI_URL when the setting is nil" do
         allow(PluginModel).to receive(:get_setting).with('pypi_url', scope: "DEFAULT").and_return(nil)
         allow(ENV).to receive(:fetch).with('PYPI_URL', nil).and_return("https://env.pypi.example.com")
diff --git a/openc3/spec/models/python_package_model_spec.rb b/openc3/spec/models/python_package_model_spec.rb
index 6df1dbf71e..dc3b3e233f 100644
--- a/openc3/spec/models/python_package_model_spec.rb
+++ b/openc3/spec/models/python_package_model_spec.rb
@@ -434,6 +434,13 @@ def install_spawn(**kwargs)
         expect(cmd).to include("https://custom.pypi.example.com/simple")
       end
 
+      it "drops a trailing slash from the pypi_url setting" do
+        allow(PythonPackageModel).to receive(:get_setting).with('pypi_url', scope: "DEFAULT").and_return("https://custom.pypi.example.com/")
+
+        cmd, _kw = install_spawn
+        expect(cmd).to include("https://custom.pypi.example.com/simple")
+      end
+
       it "falls back to ENV PYPI_URL when get_setting raises" do
         allow(ENV).to receive(:fetch).with('PYPI_URL', nil).and_return("https://env.pypi.example.com")
         allow(PythonPackageModel).to receive(:get_setting).and_raise("no redis")
diff --git a/openc3/spec/utilities/pypi_url_spec.rb b/openc3/spec/utilities/pypi_url_spec.rb
index 0365724eb0..e39eed72fb 100644
--- a/openc3/spec/utilities/pypi_url_spec.rb
+++ b/openc3/spec/utilities/pypi_url_spec.rb
@@ -16,6 +16,25 @@
 
 module OpenC3
   describe PypiUrl do
+    describe ".index_url" do
+      it "appends /simple to a base url" do
+        expect(PypiUrl.index_url("https://mirror.example.com")).to eql "https://mirror.example.com/simple"
+      end
+
+      it "drops trailing slashes before appending /simple" do
+        expect(PypiUrl.index_url("https://mirror.example.com/")).to eql "https://mirror.example.com/simple"
+        expect(PypiUrl.index_url("https://mirror.example.com/pypi//")).to eql "https://mirror.example.com/pypi/simple"
+      end
+
+      it "maps the public index with a trailing slash to DEFAULT" do
+        expect(PypiUrl.index_url("https://pypi.org/")).to eql PypiUrl::DEFAULT
+      end
+
+      it "returns nil for nil" do
+        expect(PypiUrl.index_url(nil)).to be_nil
+      end
+    end
+
     describe ".validate" do
       context "with a valid http(s) url" do
         it "returns https urls unchanged" do