diff --git a/.github/workflows/build-preview.yml b/.github/workflows/build-preview.yml
index aae7b70304f7..3dfa1374d49a 100644
--- a/.github/workflows/build-preview.yml
+++ b/.github/workflows/build-preview.yml
@@ -119,7 +119,7 @@ jobs:
       build_ref: ${{ needs.trigger.outputs.sha }}
       release_tag: ${{ needs.trigger.outputs.release_tag }}
       is_prerelease: true
-      llvm_version: '19'
+      llvm_version: '20'
 
   notify:
     name: Notify Completion
diff --git a/.github/workflows/build-reusable.yml b/.github/workflows/build-reusable.yml
index b068720f1385..b834df139a39 100644
--- a/.github/workflows/build-reusable.yml
+++ b/.github/workflows/build-reusable.yml
@@ -1,7 +1,9 @@
 name: Build WebKit (Reusable)
 
 env:
-  LLVM_VERSION: 19
+  # Use LLVM 20 for macOS - LLVM 21 has libc++ ABI compatibility issues
+  # with macOS system libc++ (missing std::__1::__hash_memory)
+  LLVM_VERSION: 20
 
 on:
   workflow_call:
@@ -21,7 +23,7 @@ on:
       llvm_version:
         description: 'LLVM version to use'
         type: string
-        default: '19'
+        default: '20'
     outputs:
       release_tag:
         description: 'The release tag that was created'
@@ -268,7 +270,7 @@ jobs:
         run: |
           "C:\Program Files\7-Zip" >> $Env:GITHUB_PATH
           scoop config use_external_7zip true
-          scoop install ninja llvm@19.1.7
+          scoop install ninja llvm@20.1.8
           Join-Path (Resolve-Path ~).Path "scoop\apps\llvm\current\bin" >> $Env:GITHUB_PATH
       - name: Install LLVM and Ninja (ARM64)
         if: matrix.platform == 'ARM64'
@@ -277,7 +279,7 @@ jobs:
           scoop config use_external_7zip true
           scoop install ninja
           # Install LLVM ARM64 from official LLVM releases
-          # Use LLVM 21 for ARM64 - has better Windows ARM64 support and fixes SEH unwind bugs
+          # Using official releases since Scoop may not have ARM64 LLVM
           $llvmVersion = "21.1.8"
           $llvmUrl = "https://github.com/llvm/llvm-project/releases/download/llvmorg-$llvmVersion/LLVM-$llvmVersion-woa64.exe"
           $llvmPath = "C:\LLVM"
diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 97a1721913f3..60f65e520038 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -9,7 +9,7 @@ on:
     inputs:
       llvm_version:
         description: "LLVM version"
-        default: "19"
+        default: "20"
 
 jobs:
   build:
@@ -22,4 +22,4 @@ jobs:
       build_ref: ${{ github.sha }}
       release_tag: autobuild-${{ github.sha }}
       is_prerelease: false
-      llvm_version: ${{ inputs.llvm_version || '19' }}
+      llvm_version: ${{ inputs.llvm_version || '20' }}
diff --git a/Dockerfile b/Dockerfile
index 3e71409ad1a8..c88d296210ce 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -3,7 +3,7 @@ ARG WEBKIT_RELEASE_TYPE=Release
 ARG CPU=native
 ARG LTO_FLAG="-flto=full -fwhole-program-vtables -fforce-emit-vtables "
 ARG RELEASE_FLAGS="-O3 -DNDEBUG=1"
-ARG LLVM_VERSION="19"
+ARG LLVM_VERSION="20"
 ARG DEFAULT_CFLAGS="-mno-omit-leaf-frame-pointer -g -fno-omit-frame-pointer -ffunction-sections -fdata-sections -faddrsig -fno-unwind-tables -fno-asynchronous-unwind-tables -DU_STATIC_IMPLEMENTATION=1 "
 ARG ENABLE_SANITIZERS=""
 
@@ -71,10 +71,10 @@ RUN update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-13 130 \
     --slave /usr/bin/gcc-nm gcc-nm /usr/bin/gcc-nm-13 \
     --slave /usr/bin/gcc-ranlib gcc-ranlib /usr/bin/gcc-ranlib-13
 
-# Install LLVM 19
+# Install LLVM 20
 RUN wget https://apt.llvm.org/llvm.sh \
     && chmod +x llvm.sh \
-    && ./llvm.sh 19 all \
+    && ./llvm.sh 20 all \
     && rm llvm.sh \
     && rm -rf /var/lib/apt/lists/*
 
diff --git a/Dockerfile.musl b/Dockerfile.musl
index a137ff9eabee..10076f996105 100644
--- a/Dockerfile.musl
+++ b/Dockerfile.musl
@@ -2,10 +2,10 @@ ARG MARCH_FLAG=""
 ARG WEBKIT_RELEASE_TYPE=Release
 ARG CPU=native
 ARG LTO_FLAG="-flto=full -fwhole-program-vtables -fforce-emit-vtables "
-ARG LLVM_VERSION="19"
+ARG LLVM_VERSION="21"
 ARG DEFAULT_CFLAGS="-mno-omit-leaf-frame-pointer -g -fno-omit-frame-pointer -ffunction-sections -fdata-sections -faddrsig -fno-unwind-tables -fno-asynchronous-unwind-tables -DU_STATIC_IMPLEMENTATION=1 "
 
-FROM alpine:3.21 as base
+FROM alpine:3.23 as base
 
 ARG MARCH_FLAG
 ARG WEBKIT_RELEASE_TYPE
@@ -15,13 +15,13 @@ ARG LLVM_VERSION
 ARG DEFAULT_CFLAGS
 
 RUN apk update
-RUN apk add --no-cache cmake make clang19 clang19-static clang19-dev llvm19-dev llvm19-static musl-dev git lld libgcc gcc g++ libstdc++ build-base lld-dev llvm19-libs libc-dev xz zlib zlib-dev libxml2 libxml2-dev
+RUN apk add --no-cache cmake make clang21 clang21-static clang21-dev llvm21-dev llvm21-static musl-dev git lld libgcc gcc g++ libstdc++ build-base lld-dev llvm21-libs libc-dev xz zlib zlib-dev libxml2 libxml2-dev
 
-ENV CXX=clang++-19
-ENV CC=clang-19
-ENV LDFLAGS='-L/usr/include -L/usr/include/llvm19'
-ENV CXXFLAGS="-I/usr/include -I/usr/include/llvm19"
-ENV PATH="/usr/bin:/usr/local/bin:/zig/bin:/usr/lib/llvm19/bin:$PATH"
+ENV CXX=clang++-21
+ENV CC=clang-21
+ENV LDFLAGS='-L/usr/include -L/usr/include/llvm21'
+ENV CXXFLAGS="-I/usr/include -I/usr/include/llvm21"
+ENV PATH="/usr/bin:/usr/local/bin:/zig/bin:/usr/lib/llvm21/bin:$PATH"
 ENV CPU=${CPU}
 ENV MARCH_FLAG=${MARCH_FLAG}
 ENV WEBKIT_OUT_DIR=/webkitbuild
@@ -66,7 +66,7 @@ ARG LTO_FLAG
 ARG LLVM_VERSION
 ARG DEFAULT_CFLAGS
 
-RUN apk add --no-cache cpio curl file gnupg ninja ruby unzip rsync perl python3 openssl-dev openssl linux-headers
+RUN apk add --no-cache cpio curl file gnupg ninja ruby ruby-getoptlong unzip rsync perl python3 openssl-dev openssl linux-headers
 
 ENV WEBKIT_OUT_DIR=/webkitbuild
 # These are unnecessary on musl