diff --git a/lib/redmine/scm/adapters/git_adapter.rb b/lib/redmine/scm/adapters/git_adapter.rb
index 192df0229..0e04d0a20 100644
--- a/lib/redmine/scm/adapters/git_adapter.rb
+++ b/lib/redmine/scm/adapters/git_adapter.rb
@@ -25,6 +25,9 @@ module Redmine
       class GitAdapter < AbstractAdapter
         # Git executable name
         GIT_BIN = Redmine::Configuration['scm_git_command'] || "git"
+        # Repositories created after 2020 may have a default branch of
+        # "main" instead of "master"
+        GIT_DEFAULT_BRANCH_NAMES = %w[main master].freeze
 
         class GitBranch < Branch
           attr_accessor :is_default
@@ -110,14 +113,13 @@ module Redmine
         end
 
         def default_branch
-          bras = self.branches
-          return unless bras
+          return if branches.blank?
 
-          default_bras = bras.detect{|x| x.is_default == true}
-          return default_bras.to_s if default_bras
-
-          master_bras = bras.detect{|x| x.to_s == 'master'}
-          master_bras ? 'master' : bras.first.to_s
+          (
+            branches.detect(&:is_default) ||
+            branches.detect {|b| GIT_DEFAULT_BRANCH_NAMES.include?(b.to_s)} ||
+            branches.first
+          ).to_s
         end
 
         def entry(path=nil, identifier=nil)
diff --git a/test/unit/lib/redmine/scm/adapters/git_adapter_test.rb b/test/unit/lib/redmine/scm/adapters/git_adapter_test.rb
index 6d849990c..4e3c570b6 100644
--- a/test/unit/lib/redmine/scm/adapters/git_adapter_test.rb
+++ b/test/unit/lib/redmine/scm/adapters/git_adapter_test.rb
@@ -124,6 +124,25 @@ class GitAdapterTest < ActiveSupport::TestCase
 
     def test_default_branch
       assert_equal 'master-20120212', @adapter.default_branch
+
+      # When no branch is marked as the default, GitAdapter treats
+      # "main" or "master" branch as the default
+      b_foo, b_bar, b_main, b_master =
+        %w[foo bar main master].map do |name|
+          Redmine::Scm::Adapters::GitAdapter::GitBranch.new(name)
+        end
+      @adapter.stubs(:branches).returns([b_foo, b_main, b_bar])
+      assert_equal 'main', @adapter.default_branch
+      @adapter.stubs(:branches).returns([b_foo, b_master, b_bar])
+      assert_equal 'master', @adapter.default_branch
+
+      # The first found branch is treated as the default branch
+      # when neither "main" nor "master" is found
+      @adapter.stubs(:branches).returns([b_foo, b_bar])
+      assert_equal 'foo', @adapter.default_branch
+
+      @adapter.stubs(:branches).returns([])
+      assert_nil @adapter.default_branch
     end
 
     def test_tags