diff --git a/.github/workflows/scripts/windows/swift/install-swift-nightly-6.4.x.ps1 b/.github/workflows/scripts/windows/swift/install-swift-nightly-6.4.x.ps1
new file mode 100644
index 00000000..410bbe27
--- /dev/null
+++ b/.github/workflows/scripts/windows/swift/install-swift-nightly-6.4.x.ps1
@@ -0,0 +1,24 @@
+##===----------------------------------------------------------------------===##
+##
+## This source file is part of the Swift.org open source project
+##
+## Copyright (c) 2026 Apple Inc. and the Swift project authors
+## Licensed under Apache License v2.0 with Runtime Library Exception
+##
+## See https://swift.org/LICENSE.txt for license information
+## See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+##
+##===----------------------------------------------------------------------===##
+. $PSScriptRoot\install-swift.ps1
+
+if ([System.Runtime.InteropServices.RuntimeInformation]::OSArchitecture -eq "Arm64") {
+    $swiftOSVersion = 'windows10-arm64'
+} else {
+    $swiftOSVersion = 'windows10'
+}
+
+$SWIFT_RELEASE_METADATA="https://download.swift.org/swift-6.4.x-branch/$swiftOSVersion/latest-build.json"
+$Release = curl.exe -sL ${SWIFT_RELEASE_METADATA}
+$SWIFT_URL = "https://download.swift.org/swift-6.4.x-branch/$swiftOSVersion/$($($Release | ConvertFrom-JSON).dir)/$($($Release | ConvertFrom-JSON).download)"
+
+Install-Swift -Url $SWIFT_URL -Sha256 ""
diff --git a/.github/workflows/swift_package_test.yml b/.github/workflows/swift_package_test.yml
index cef8d3fb..e8a7905c 100644
--- a/.github/workflows/swift_package_test.yml
+++ b/.github/workflows/swift_package_test.yml
@@ -41,7 +41,7 @@ on:
       linux_swift_versions:
         type: string
         description: "Include Linux Swift version list (JSON)"
-        default: "[ \"5.9\", \"5.10\", \"6.0\", \"6.1\", \"6.2\", \"6.3\", \"nightly-main\", \"nightly-6.3\"]"
+        default: "[ \"5.9\", \"5.10\", \"6.0\", \"6.1\", \"6.2\", \"6.3\", \"nightly-main\", \"nightly-6.4.x\"]"
       linux_exclude_swift_versions:
         type: string
         description: "Exclude Linux Swift version list (JSON)"
@@ -57,7 +57,7 @@ on:
       linux_static_sdk_versions:
         type: string
         description: "Static Linux Swift SDK version list (JSON)"
-        default: "[\"nightly-main\", \"nightly-6.3\", \"6.3\"]"
+        default: "[\"nightly-main\", \"nightly-6.4.x\", \"6.3\"]"
       linux_static_sdk_exclude_swift_versions:
         type: string
         description: "Exclude Static Linux Swift SDK version list (JSON)"
@@ -65,7 +65,7 @@ on:
       wasm_sdk_versions:
         type: string
         description: "Wasm Swift SDK version list (JSON)"
-        default: "[\"nightly-main\", \"nightly-6.3\", \"6.3\"]"
+        default: "[\"nightly-main\", \"nightly-6.4.x\", \"6.3\"]"
       wasm_exclude_swift_versions:
         type: string
         description: "Exclude Wasm Swift SDK version list (JSON)"
@@ -73,7 +73,7 @@ on:
       android_sdk_versions:
         type: string
         description: "Android Swift SDK version list (JSON)"
-        default: "[\"nightly-main\", \"nightly-6.3\", \"6.3\"]"
+        default: "[\"nightly-main\", \"nightly-6.4.x\", \"6.3\"]"
       android_exclude_swift_versions:
         type: string
         description: "Exclude Android Swift SDK version list (JSON)"
@@ -82,7 +82,7 @@ on:
         type: string
         description: "Include Windows Swift version list (JSON)"
         # "5.10" is omitted for Windows because the container image is broken.
-        default: "[\"5.9\", \"6.0\", \"6.1\", \"6.2\", \"6.3\", \"nightly-main\", \"nightly-6.3\"]"
+        default: "[\"5.9\", \"6.0\", \"6.1\", \"6.2\", \"6.3\", \"nightly-main\", \"nightly-6.4.x\"]"
       windows_exclude_swift_versions:
         type: string
         description: "Exclude Windows Swift version list (JSON)"