diff --git a/Sources/SwiftDriver/Toolchains/DarwinToolchain.swift b/Sources/SwiftDriver/Toolchains/DarwinToolchain.swift
index f9de16dba..79bce9cc2 100644
--- a/Sources/SwiftDriver/Toolchains/DarwinToolchain.swift
+++ b/Sources/SwiftDriver/Toolchains/DarwinToolchain.swift
@@ -456,29 +456,53 @@ public final class DarwinToolchain: Toolchain {
     }
 
     if driver.isFrontendArgSupported(.externalPluginPath) {
-      // Determine the platform path. For simulator platforms, look into the
-      // corresponding device platform instance.
-      let origPlatformPath = VirtualPath.lookup(sdkPath)
-        .parentDirectory
-        .parentDirectory
-        .parentDirectory
-      let platformPath: VirtualPath
-      if let simulatorRange = origPlatformPath.basename.range(of: "Simulator.platform") {
-        let devicePlatform = origPlatformPath.basename.replacingCharacters(in: simulatorRange, with: "OS.platform")
-        platformPath = origPlatformPath.parentDirectory.appending(component: devicePlatform)
-      } else {
-        platformPath = origPlatformPath
+      // If the PLATFORM_DIR environment variable is set, also add plugin
+      // paths into it. Since this is a user override, it comes beore the
+      // default platform path that's based on the SDK.
+      if let platformDir = env["PLATFORM_DIR"],
+         let platformPath = try? VirtualPath(path: platformDir) {
+        addPluginPaths(
+          forPlatform: platformPath,
+          commandLine: &commandLine
+        )
       }
 
-      // Default paths for compiler plugins within the platform (accessed via that
-      // platform's plugin server).
-      let platformPathRoot = platformPath.appending(components: "Developer", "usr")
-      commandLine.appendFlag(.externalPluginPath)
-      commandLine.appendFlag("\(platformPathRoot.pluginPath.name)#\(platformPathRoot.pluginServerPath.name)")
+      // Determine the platform path based on the SDK path.
+      addPluginPaths(
+        forPlatform: VirtualPath.lookup(sdkPath)
+          .parentDirectory
+          .parentDirectory
+          .parentDirectory,
+        commandLine: &commandLine
+      )
+    }
+  }
 
-      commandLine.appendFlag(.externalPluginPath)
-      commandLine.appendFlag("\(platformPathRoot.localPluginPath.name)#\(platformPathRoot.pluginServerPath.name)")
+  /// Given the platform path (e.g., a path into something like iPhoneOS.platform),
+  /// add external plugin path arguments for compiler plugins that are distributed
+  /// within that path.
+  func addPluginPaths(
+    forPlatform origPlatformPath: VirtualPath,
+    commandLine: inout [Job.ArgTemplate]
+  ) {
+    // For simulator platforms, look into the corresponding device platform instance,
+    // because they share compiler plugins.
+    let platformPath: VirtualPath
+    if let simulatorRange = origPlatformPath.basename.range(of: "Simulator.platform") {
+      let devicePlatform = origPlatformPath.basename.replacingCharacters(in: simulatorRange, with: "OS.platform")
+      platformPath = origPlatformPath.parentDirectory.appending(component: devicePlatform)
+    } else {
+      platformPath = origPlatformPath
     }
+
+    // Default paths for compiler plugins within the platform (accessed via that
+    // platform's plugin server).
+    let platformPathRoot = platformPath.appending(components: "Developer", "usr")
+    commandLine.appendFlag(.externalPluginPath)
+    commandLine.appendFlag("\(platformPathRoot.pluginPath.name)#\(platformPathRoot.pluginServerPath.name)")
+
+    commandLine.appendFlag(.externalPluginPath)
+    commandLine.appendFlag("\(platformPathRoot.localPluginPath.name)#\(platformPathRoot.pluginServerPath.name)")
   }
 }
 
diff --git a/Tests/SwiftDriverTests/SwiftDriverTests.swift b/Tests/SwiftDriverTests/SwiftDriverTests.swift
index ac7b95bbe..c1a2c413d 100644
--- a/Tests/SwiftDriverTests/SwiftDriverTests.swift
+++ b/Tests/SwiftDriverTests/SwiftDriverTests.swift
@@ -7749,7 +7749,12 @@ final class SwiftDriverTests: XCTestCase {
   func pluginPathTest(platform: String, sdk: String, searchPlatform: String) throws {
     let sdkRoot = try testInputsPath.appending(
       components: ["Platform Checks", "\(platform).platform", "Developer", "SDKs", "\(sdk).sdk"])
-    var driver = try Driver(args: ["swiftc", "-typecheck", "foo.swift", "-sdk", VirtualPath.absolute(sdkRoot).name, "-plugin-path", "PluginA", "-external-plugin-path", "Plugin~B#Bexe", "-load-plugin-library", "PluginB2", "-plugin-path", "PluginC", "-working-directory", "/tmp"])
+    var env = ProcessEnv.vars
+    env["PLATFORM_DIR"] = "/tmp/PlatformDir/\(platform).platform"
+    var driver = try Driver(
+      args: ["swiftc", "-typecheck", "foo.swift", "-sdk", VirtualPath.absolute(sdkRoot).name, "-plugin-path", "PluginA", "-external-plugin-path", "Plugin~B#Bexe", "-load-plugin-library", "PluginB2", "-plugin-path", "PluginC", "-working-directory", "/tmp"],
+      env: env
+    )
     guard driver.isFrontendArgSupported(.pluginPath) && driver.isFrontendArgSupported(.externalPluginPath) else {
       return
     }
@@ -7791,6 +7796,18 @@ final class SwiftDriverTests: XCTestCase {
     XCTAssertNotNil(platformLocalPluginPathIndex)
     XCTAssertLessThan(platformPluginPathIndex!, platformLocalPluginPathIndex!)
 
+    // Plugin paths that come from the PLATFORM_DIR environment variable.
+    let envOrigPlatformPath = try AbsolutePath(validating: "/tmp/PlatformDir/\(searchPlatform).platform")
+    let envPlatformPath = envOrigPlatformPath.appending(components: "Developer", "usr")
+    let envPlatformServerPath = envPlatformPath.appending(components: "bin", "swift-plugin-server").pathString
+    let envPlatformPluginPath = envPlatformPath.appending(components: "lib", "swift", "host", "plugins")
+    let envPlatformPluginPathIndex = job.commandLine.firstIndex(of: .flag("\(envPlatformPluginPath)#\(envPlatformServerPath)"))
+    XCTAssertNotNil(envPlatformPluginPathIndex)
+
+    if let platformPluginPathIndex, let envPlatformPluginPathIndex {
+      XCTAssertLessThan(envPlatformPluginPathIndex, platformPluginPathIndex)
+    }
+
     let toolchainPluginPathIndex = job.commandLine.firstIndex(of: .path(.absolute(try driver.toolchain.executableDir.parentDirectory.appending(components: "lib", "swift", "host", "plugins"))))
     XCTAssertNotNil(toolchainPluginPathIndex)