diff --git a/Sources/LanguageServerProtocol/SupportTypes/DocumentURI.swift b/Sources/LanguageServerProtocol/SupportTypes/DocumentURI.swift
index 5f593d747..ad50fb849 100644
--- a/Sources/LanguageServerProtocol/SupportTypes/DocumentURI.swift
+++ b/Sources/LanguageServerProtocol/SupportTypes/DocumentURI.swift
@@ -56,12 +56,21 @@ public struct DocumentURI: Codable, Hashable, Sendable {
   /// fallback mode that drops semantic functionality.
   public var pseudoPath: String {
     if storage.isFileURL {
-      return storage.withUnsafeFileSystemRepresentation { filePath in
-        if let filePath {
-          String(cString: filePath)
-        } else {
-          ""
+      return storage.withUnsafeFileSystemRepresentation { filePathPtr in
+        guard let filePathPtr else {
+          return ""
         }
+        let filePath = String(cString: filePathPtr)
+        #if os(Windows)
+        // VS Code spells file paths with a lowercase drive letter, while the rest of Windows APIs use an uppercase
+        // drive letter. Normalize the drive letter spelling to be uppercase.
+        if filePath.first?.isASCII ?? false, filePath.first?.isLetter ?? false, filePath.first?.isLowercase ?? false,
+          filePath.count > 1, filePath[filePath.index(filePath.startIndex, offsetBy: 1)] == ":"
+        {
+          return filePath.first!.uppercased() + filePath.dropFirst()
+        }
+        #endif
+        return filePath
       }
     } else {
       return storage.absoluteString
diff --git a/Sources/SKTestSupport/SkipUnless.swift b/Sources/SKTestSupport/SkipUnless.swift
index dbfe49777..69a9deb5b 100644
--- a/Sources/SKTestSupport/SkipUnless.swift
+++ b/Sources/SKTestSupport/SkipUnless.swift
@@ -418,6 +418,10 @@ package actor SkipUnless {
     try XCTSkipUnless(Platform.current == .darwin, message)
   }
 
+  package static func platformIsWindows(_ message: String) throws {
+    try XCTSkipUnless(Platform.current == .windows, message)
+  }
+
   package static func platformSupportsTaskPriorityElevation() throws {
     #if os(macOS)
     guard #available(macOS 14.0, *) else {
diff --git a/Sources/SwiftExtensions/URLExtensions.swift b/Sources/SwiftExtensions/URLExtensions.swift
index 024e91f6f..cfac5ae81 100644
--- a/Sources/SwiftExtensions/URLExtensions.swift
+++ b/Sources/SwiftExtensions/URLExtensions.swift
@@ -67,11 +67,21 @@ extension URL {
       guard self.isFileURL else {
         throw FilePathError.noFileURL(self)
       }
-      return try self.withUnsafeFileSystemRepresentation { buffer in
-        guard let buffer else {
+      return try self.withUnsafeFileSystemRepresentation { filePathPtr in
+        guard let filePathPtr else {
           throw FilePathError.noFileSystemRepresentation(self)
         }
-        return String(cString: buffer)
+        let filePath = String(cString: filePathPtr)
+        #if os(Windows)
+        // VS Code spells file paths with a lowercase drive letter, while the rest of Windows APIs use an uppercase
+        // drive letter. Normalize the drive letter spelling to be uppercase.
+        if filePath.first?.isASCII ?? false, filePath.first?.isLetter ?? false, filePath.first?.isLowercase ?? false,
+          filePath.count > 1, filePath[filePath.index(filePath.startIndex, offsetBy: 1)] == ":"
+        {
+          return filePath.first!.uppercased() + filePath.dropFirst()
+        }
+        #endif
+        return filePath
       }
     }
   }
diff --git a/Tests/SourceKitLSPTests/PullDiagnosticsTests.swift b/Tests/SourceKitLSPTests/PullDiagnosticsTests.swift
index 51ca9fb18..bcbcd2c11 100644
--- a/Tests/SourceKitLSPTests/PullDiagnosticsTests.swift
+++ b/Tests/SourceKitLSPTests/PullDiagnosticsTests.swift
@@ -48,6 +48,35 @@ final class PullDiagnosticsTests: XCTestCase {
     XCTAssertEqual(diagnostic.range, Position(line: 1, utf16index: 2)..<Position(line: 1, utf16index: 9))
   }
 
+  func testDiagnosticsIfFileIsOpenedWithLowercaseDriveLetter() async throws {
+    try SkipUnless.platformIsWindows("Drive letters only exist on Windows")
+
+    let fileContents = """
+      func foo() {
+        invalid
+      }
+      """
+
+    // We use `IndexedSingleSwiftFileTestProject` so that the test file exists on disk, which causes sourcekitd to
+    // uppercase the drive letter.
+    let project = try await IndexedSingleSwiftFileTestProject(fileContents, allowBuildFailure: true)
+    project.testClient.send(DidCloseTextDocumentNotification(textDocument: TextDocumentIdentifier(project.fileURI)))
+
+    let filePath = try XCTUnwrap(project.fileURI.fileURL?.filePath)
+    XCTAssertEqual(filePath[filePath.index(filePath.startIndex, offsetBy: 1)], ":")
+    let lowercaseDriveLetterPath = filePath.first!.lowercased() + filePath.dropFirst()
+    let uri = DocumentURI(filePath: lowercaseDriveLetterPath, isDirectory: false)
+    project.testClient.openDocument(fileContents, uri: uri)
+
+    let report = try await project.testClient.send(
+      DocumentDiagnosticsRequest(textDocument: TextDocumentIdentifier(uri))
+    )
+
+    XCTAssertEqual(report.fullReport?.items.count, 1)
+    let diagnostic = try XCTUnwrap(report.fullReport?.items.first)
+    XCTAssertEqual(diagnostic.range, Position(line: 1, utf16index: 2)..<Position(line: 1, utf16index: 9))
+  }
+
   /// Test that we can get code actions for pulled diagnostics (https://github.com/swiftlang/sourcekit-lsp/issues/776)
   func testCodeActions() async throws {
     let testClient = try await TestSourceKitLSPClient(