diff --git a/.devcontainer/Dockerfile b/.devcontainer/Dockerfile
index a7a8e26d91e..97e1f4036d6 100644
--- a/.devcontainer/Dockerfile
+++ b/.devcontainer/Dockerfile
@@ -1,5 +1,5 @@
 # For details, see https://github.com/devcontainers/images/tree/main/src/ruby
-FROM mcr.microsoft.com/devcontainers/ruby:3.4-trixie
+FROM mcr.microsoft.com/devcontainers/ruby:4.0-trixie
 
 # Install node version from .nvmrc
 WORKDIR /app
diff --git a/.github/ISSUE_TEMPLATE/2.server_bug_report.yml b/.github/ISSUE_TEMPLATE/2.server_bug_report.yml
index ec3a52689d4..467df3ef7d9 100644
--- a/.github/ISSUE_TEMPLATE/2.server_bug_report.yml
+++ b/.github/ISSUE_TEMPLATE/2.server_bug_report.yml
@@ -59,7 +59,7 @@ body:
         Any additional technical details you may have, like logs or error traces
       value: |
         If this is happening on your own Mastodon server, please fill out those:
-        - Ruby version: (from `ruby --version`, eg. v3.4.9)
+        - Ruby version: (from `ruby --version`, eg. v4.0.2)
         - Node.js version: (from `node --version`, eg. v22.16.0)
     validations:
       required: false
diff --git a/.github/ISSUE_TEMPLATE/3.troubleshooting.yml b/.github/ISSUE_TEMPLATE/3.troubleshooting.yml
index 5278933ecef..9210e99b438 100644
--- a/.github/ISSUE_TEMPLATE/3.troubleshooting.yml
+++ b/.github/ISSUE_TEMPLATE/3.troubleshooting.yml
@@ -61,7 +61,7 @@ body:
       value: |
         Please at least include those informations:
         - Operating system: (eg. Ubuntu 24.04.2)
-        - Ruby version: (from `ruby --version`, eg. v3.4.9)
+        - Ruby version: (from `ruby --version`, eg. v4.0.2)
         - Node.js version: (from `node --version`, eg. v22.16.0)
     validations:
       required: false
diff --git a/.github/actions/setup-ruby/action.yml b/.github/actions/setup-ruby/action.yml
index 2993c66f7bc..9f50aa60ffa 100644
--- a/.github/actions/setup-ruby/action.yml
+++ b/.github/actions/setup-ruby/action.yml
@@ -1,4 +1,4 @@
-name: 'Setup RUby'
+name: 'Setup Ruby'
 description: 'Setup a Ruby environment ready to run the Mastodon code'
 inputs:
   ruby-version:
@@ -17,7 +17,7 @@ runs:
         sudo apt-get install --no-install-recommends -y libicu-dev libidn11-dev libvips42 ${{ inputs.additional-system-dependencies }}
 
     - name: Set up Ruby
-      uses: ruby/setup-ruby@4eb9f110bac952a8b68ecf92e3b5c7a987594ba6 # v1
+      uses: ruby/setup-ruby@c984c1a20bb35a1cbda04477c816cea024418be9 # v1
       with:
         ruby-version: ${{ inputs.ruby-version }}
         bundler-cache: true
diff --git a/.github/workflows/bundler-audit.yml b/.github/workflows/bundler-audit.yml
index b1e8d406e25..964d93378d3 100644
--- a/.github/workflows/bundler-audit.yml
+++ b/.github/workflows/bundler-audit.yml
@@ -31,7 +31,7 @@ jobs:
         uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
 
       - name: Set up Ruby
-        uses: ruby/setup-ruby@4eb9f110bac952a8b68ecf92e3b5c7a987594ba6 # v1
+        uses: ruby/setup-ruby@c984c1a20bb35a1cbda04477c816cea024418be9 # v1
         with:
           bundler-cache: true
 
diff --git a/.github/workflows/lint-haml.yml b/.github/workflows/lint-haml.yml
index 6cc212180a8..33dc4e4fb64 100644
--- a/.github/workflows/lint-haml.yml
+++ b/.github/workflows/lint-haml.yml
@@ -36,7 +36,7 @@ jobs:
         uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
 
       - name: Set up Ruby
-        uses: ruby/setup-ruby@4eb9f110bac952a8b68ecf92e3b5c7a987594ba6 # v1
+        uses: ruby/setup-ruby@c984c1a20bb35a1cbda04477c816cea024418be9 # v1
         with:
           bundler-cache: true
 
diff --git a/.github/workflows/lint-ruby.yml b/.github/workflows/lint-ruby.yml
index 216618f4e56..06e4762d114 100644
--- a/.github/workflows/lint-ruby.yml
+++ b/.github/workflows/lint-ruby.yml
@@ -38,7 +38,7 @@ jobs:
         uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
 
       - name: Set up Ruby
-        uses: ruby/setup-ruby@4eb9f110bac952a8b68ecf92e3b5c7a987594ba6 # v1
+        uses: ruby/setup-ruby@c984c1a20bb35a1cbda04477c816cea024418be9 # v1
         with:
           bundler-cache: true
 
diff --git a/.github/workflows/test-ruby.yml b/.github/workflows/test-ruby.yml
index d0c1d4fbcd9..a2a496475f4 100644
--- a/.github/workflows/test-ruby.yml
+++ b/.github/workflows/test-ruby.yml
@@ -125,6 +125,7 @@ jobs:
       matrix:
         ruby-version:
           - '3.3'
+          - '3.4'
           - '.ruby-version'
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
@@ -217,6 +218,7 @@ jobs:
       matrix:
         ruby-version:
           - '3.3'
+          - '3.4'
           - '.ruby-version'
 
     steps:
@@ -347,6 +349,7 @@ jobs:
       matrix:
         ruby-version:
           - '3.3'
+          - '3.4'
           - '.ruby-version'
         search-image:
           - docker.elastic.co/elasticsearch/elasticsearch:7.17.29
diff --git a/.ruby-version b/.ruby-version
index 7bcbb3808b5..4d54daddb61 100644
--- a/.ruby-version
+++ b/.ruby-version
@@ -1 +1 @@
-3.4.9
+4.0.2
diff --git a/Dockerfile b/Dockerfile
index c26f957cee2..df02e48104d 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -11,9 +11,9 @@ ARG TARGETPLATFORM=${TARGETPLATFORM}
 ARG BUILDPLATFORM=${BUILDPLATFORM}
 ARG BASE_REGISTRY="docker.io"
 
-# Ruby image to use for base image, change with [--build-arg RUBY_VERSION="3.4.x"]
+# Ruby image to use for base image, change with [--build-arg RUBY_VERSION="4.0.x"]
 # renovate: datasource=docker depName=docker.io/ruby
-ARG RUBY_VERSION="3.4.9"
+ARG RUBY_VERSION="4.0.2"
 # # Node.js version to use in base image, change with [--build-arg NODE_MAJOR_VERSION="22"]
 # renovate: datasource=node-version depName=node
 ARG NODE_MAJOR_VERSION="24"
diff --git a/Gemfile b/Gemfile
index 8f283db728d..44b2b3d1025 100644
--- a/Gemfile
+++ b/Gemfile
@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 source 'https://rubygems.org'
-ruby '>= 3.3.0', '< 3.5.0'
+ruby '>= 3.3.0', '< 4.1.0'
 
 gem 'propshaft'
 gem 'puma', '~> 7.0'
diff --git a/Gemfile.lock b/Gemfile.lock
index d5d71953b40..92cc117b1cf 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -1097,7 +1097,7 @@ DEPENDENCIES
   xorcist (~> 1.1)
 
 RUBY VERSION
-  ruby 3.4.9
+  ruby 4.0.2
 
 BUNDLED WITH
   4.0.8