diff --git a/Library/Homebrew/bundle/brewfile.rb b/Library/Homebrew/bundle/brewfile.rb
index 992ce4fe03..2b139e4900 100644
--- a/Library/Homebrew/bundle/brewfile.rb
+++ b/Library/Homebrew/bundle/brewfile.rb
@@ -24,16 +24,22 @@ module Homebrew
           else
             raise "'HOMEBREW_BUNDLE_FILE' cannot be specified with '--global'" if env_bundle_file.present?
 
-            if user_config_home
-              xdg_path = "#{user_config_home}/Brewfile"
-              legacy_path = Bundle.exchange_uid_if_needed! { "#{Dir.home}/.Brewfile" }
-
-              # Prefer existing file to maintain backwards compatibility
-              if File.exist?(xdg_path) || !File.exist?(legacy_path)
-                xdg_path
+            # Determine if we're in XDG mode by checking if user_config_home is XDG-based
+            # In bin/brew: XDG_CONFIG_HOME set -> HOMEBREW_USER_CONFIG_HOME="${XDG_CONFIG_HOME}/homebrew"
+            # XDG_CONFIG_HOME not set -> HOMEBREW_USER_CONFIG_HOME="${HOME}/.homebrew"
+            using_xdg = user_config_home&.end_with?("/homebrew")
+            
+            if using_xdg && user_config_home
+              # XDG mode: check both XDG and legacy locations, preferring existing files
+              xdg_brewfile = "#{user_config_home}/Brewfile"
+              if File.exist?(xdg_brewfile)
+                xdg_brewfile
               else
-                legacy_path
+                legacy_brewfile = Bundle.exchange_uid_if_needed! { "#{Dir.home}/.Brewfile" }
+                File.exist?(legacy_brewfile) ? legacy_brewfile : xdg_brewfile
               end
+            elsif user_config_home && File.exist?("#{user_config_home}/Brewfile")
+              "#{user_config_home}/Brewfile"
             else
               Bundle.exchange_uid_if_needed! do
                 "#{Dir.home}/.Brewfile"
diff --git a/Library/Homebrew/test/bundle/brewfile_spec.rb b/Library/Homebrew/test/bundle/brewfile_spec.rb
index 4b08323418..279def04b9 100644
--- a/Library/Homebrew/test/bundle/brewfile_spec.rb
+++ b/Library/Homebrew/test/bundle/brewfile_spec.rb
@@ -26,8 +26,11 @@ RSpec.describe Homebrew::Bundle::Brewfile do
 
       allow(ENV).to receive(:fetch).with("HOMEBREW_USER_CONFIG_HOME", any_args)
                                    .and_return(env_user_config_home_value)
+      allow(ENV).to receive(:[]).with("XDG_CONFIG_HOME").and_return(nil)
       allow(File).to receive(:exist?).with("/Users/username/.homebrew/Brewfile")
                                      .and_return(config_dir_brewfile_exist)
+      # Allow any other File.exist? calls to return false by default
+      allow(File).to receive(:exist?).and_return(false)
     end
 
     context "when `file` is specified with a relative path" do
@@ -175,14 +178,27 @@ RSpec.describe Homebrew::Bundle::Brewfile do
       context "when HOMEBREW_USER_CONFIG_HOME is set but neither Brewfile exists" do
         let(:config_dir_brewfile_exist) { false }
 
-        before do
-          # Mock that both XDG and legacy Brewfiles don't exist
-          allow(File).to receive(:exist?).with("/Users/username/.homebrew/Brewfile").and_return(false)
-          allow(File).to receive(:exist?).with("#{Dir.home}/.Brewfile").and_return(false)
+        context "when XDG_CONFIG_HOME is set" do
+          before do
+            allow(ENV).to receive(:[]).with("XDG_CONFIG_HOME").and_return("/custom/xdg")
+            # Mock that both XDG and legacy Brewfiles don't exist
+            allow(File).to receive(:exist?).with("/Users/username/.homebrew/Brewfile").and_return(false)
+            allow(File).to receive(:exist?).with("#{Dir.home}/.Brewfile").and_return(false)
+          end
+
+          it "returns the XDG path for initial Brewfile creation" do
+            expect(path).to eq(Pathname.new("#{env_user_config_home_value}/Brewfile"))
+          end
         end
 
-        it "returns the XDG path for initial Brewfile creation" do
-          expect(path).to eq(Pathname.new("#{env_user_config_home_value}/Brewfile"))
+        context "when XDG_CONFIG_HOME is not set" do
+          before do
+            allow(ENV).to receive(:[]).with("XDG_CONFIG_HOME").and_return(nil)
+          end
+
+          it "returns the legacy path" do
+            expect(path).to eq(Pathname.new("#{Dir.home}/.Brewfile"))
+          end
         end
       end
     end