diff --git a/Changelog.md b/Changelog.md
index 6bd2cff4..02be5a28 100644
--- a/Changelog.md
+++ b/Changelog.md
@@ -6,6 +6,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+## v3.22.0
+
+### Fixed
+
+Make connection switching thread safe, by fixing a thread safety issue caused by using a (class) instance variable instead of a thread-local variable.
+
 ## v3.21.0
 
 ### Added
diff --git a/active_record_shards.gemspec b/active_record_shards.gemspec
index 0f342c34..3aef9ed0 100644
--- a/active_record_shards.gemspec
+++ b/active_record_shards.gemspec
@@ -1,4 +1,4 @@
-Gem::Specification.new "active_record_shards", "3.21.0" do |s|
+Gem::Specification.new "active_record_shards", "3.22.0" do |s|
   s.authors     = ["Benjamin Quorning", "Gabe Martin-Dempesy", "Pierre Schambacher", "Mick Staugaard", "Eric Chapweske", "Ben Osheroff"]
   s.email       = ["bquorning@zendesk.com", "gabe@zendesk.com", "pschambacher@zendesk.com", "mick@staugaard.com"]
   s.homepage    = "https://github.com/zendesk/active_record_shards"
diff --git a/lib/active_record_shards/connection_switcher.rb b/lib/active_record_shards/connection_switcher.rb
index ccb2cdca..86496664 100644
--- a/lib/active_record_shards/connection_switcher.rb
+++ b/lib/active_record_shards/connection_switcher.rb
@@ -113,12 +113,11 @@ def on_primary(&block)
     alias_method :with_slave_unless, :on_replica_unless
 
     def on_cx_switch_block(which, force: false, construct_ro_scope: nil, &block)
-      @disallow_replica ||= 0
-      @disallow_replica += 1 if [:primary, :master].include?(which)
+      self.disallow_replica += 1 if [:primary, :master].include?(which)
 
       ActiveRecordShards::Deprecation.warn('the `:master` option should be replaced with `:primary`!') if which == :master
 
-      switch_to_replica = force || @disallow_replica.zero?
+      switch_to_replica = force || disallow_replica.zero?
       old_options = current_shard_selection.options
 
       switch_connection(replica: switch_to_replica)
@@ -131,10 +130,18 @@ def on_cx_switch_block(which, force: false, construct_ro_scope: nil, &block)
         readonly.scoping(&block)
       end
     ensure
-      @disallow_replica -= 1 if [:primary, :master].include?(which)
+      self.disallow_replica -= 1 if [:primary, :master].include?(which)
       switch_connection(old_options) if old_options
     end
 
+    def disallow_replica=(value)
+      Thread.current[:__active_record_shards__disallow_replica_by_thread] = value
+    end
+
+    def disallow_replica
+      Thread.current[:__active_record_shards__disallow_replica_by_thread] ||= 0
+    end
+
     def supports_sharding?
       shard_names.any?
     end
diff --git a/test/database.yml b/test/database.yml
index 07386391..cf8a4f4e 100644
--- a/test/database.yml
+++ b/test/database.yml
@@ -8,6 +8,7 @@ mysql: &MYSQL
   port: <%= mysql.port %>
   password: <%= mysql.password %>
   ssl_mode: :disabled
+  reaping_frequency: 0 # Prevents ActiveRecord from spawning reaping threads.
 
 # We connect to the unsharded primary database on a different port, via a proxy,
 # so we can make the connection unavailable when testing on_replica_by_default
diff --git a/test/helper.rb b/test/helper.rb
index ce44b602..37117907 100644
--- a/test/helper.rb
+++ b/test/helper.rb
@@ -1,5 +1,9 @@
 # frozen_string_literal: true
 
+# Stop Minitest creating threads we won't use.
+# They add noise to the thread safety tests when inspecting `Thread.list`.
+ENV["MT_CPU"] ||= "1"
+
 require 'bundler/setup'
 require 'minitest/autorun'
 require 'minitest/rg'
diff --git a/test/thread_safety_test.rb b/test/thread_safety_test.rb
new file mode 100644
index 00000000..1c0467cb
--- /dev/null
+++ b/test/thread_safety_test.rb
@@ -0,0 +1,273 @@
+# frozen_string_literal: true
+
+require_relative 'helper'
+require_relative 'models'
+
+describe "connection switching thread safety" do
+  with_fresh_databases
+
+  before do
+    ActiveRecord::Base.establish_connection(:test)
+    use_same_connection_handler_for_all_theads
+    create_seed_data
+  end
+
+  after do
+    ActiveRecord::Base.connection_handler.clear_all_connections!
+  end
+
+  it "can safely switch between all database connections in parallel" do
+    new_thread("switches_through_all_1") do
+      pause_and_mark_ready
+      switch_through_all_databases
+    end
+    new_thread("switches_through_all_2") do
+      pause_and_mark_ready
+      switch_through_all_databases
+    end
+    new_thread("switches_through_all_3") do
+      pause_and_mark_ready
+      switch_through_all_databases
+    end
+
+    wait_for_threads_to_be_ready
+    execute_and_wait_for_threads
+  end
+
+  describe "when multiple threads use different databases" do
+    it "allows threads to parallelize their IO" do
+      results = []
+
+      query_delay = { fast: "0.01", slow: "1", medium: "0.5" }
+      new_thread("different_db_parallel_thread1") do
+        ActiveRecord::Base.on_primary do
+          pause_and_mark_ready
+          result = execute_sql("SELECT name,'slower query',SLEEP(#{query_delay.fetch(:slow)}) FROM accounts")
+          assert_equal('Primary account', result.first[0])
+          results.push(result)
+        end
+      end
+
+      new_thread("different_db_parallel_thread2") do
+        ActiveRecord::Base.on_replica do
+          pause_and_mark_ready
+          result = execute_sql("SELECT name, 'faster query',SLEEP(#{query_delay.fetch(:fast)}) FROM accounts")
+          assert_equal('Replica account', result.first[0])
+          results.push(result)
+        end
+      end
+
+      new_thread("different_db_parallel_thread3") do
+        ActiveRecord::Base.on_shard(0) do
+          pause_and_mark_ready
+          result = execute_sql("SELECT title, 'medium query',SLEEP(#{query_delay.fetch(:medium)}) FROM tickets")
+          assert_equal('Shard 0 Primary ticket', result.first[0])
+          results.push(result)
+        end
+      end
+
+      wait_for_threads_to_be_ready
+
+      thread_exection_time = Benchmark.realtime do
+        execute_and_wait_for_threads
+      end
+
+      minimum_serial_query_exection_time = query_delay.values.map(&:to_f).sum
+      # Arbitrarily faster time such that there must have been some parallelization
+      max_parallel_time = minimum_serial_query_exection_time - 0.1
+      assert_operator(max_parallel_time, :>, thread_exection_time)
+
+      # This order cannot be guaranteed but it likely given the artificial delays
+      rows = results.map(&:first)
+      result_strings = rows.map { |r| r[1] }
+      assert_equal(
+        [
+          "faster query",
+          "medium query",
+          "slower query"
+        ],
+        result_strings
+      )
+    end
+  end
+
+  describe "when multiple threads use the same database" do
+    it "exposes a different connections to each thread" do
+      connections = []
+
+      new_thread("connection_per_thread1") do
+        ActiveRecord::Base.on_primary do
+          pause_and_mark_ready
+          connections << ActiveRecord::Base.connection
+        end
+      end
+
+      new_thread("connection_per_thread2") do
+        ActiveRecord::Base.on_primary do
+          pause_and_mark_ready
+          connections << ActiveRecord::Base.connection
+        end
+      end
+
+      wait_for_threads_to_be_ready
+      execute_and_wait_for_threads
+
+      expect(connections.first).must_be_kind_of(ActiveRecord::ConnectionAdapters::Mysql2Adapter)
+      assert_equal(2, connections.uniq.size)
+    end
+
+    it "allows threads to parallelize their IO" do
+      results = []
+
+      query_delay = { fast: "0.01", slow: "1", medium: "0.5" }
+      new_thread("same_db_parallel_thread1") do
+        ActiveRecord::Base.on_primary do
+          pause_and_mark_ready
+          result = execute_sql("SELECT 'slower query',SLEEP(#{query_delay.fetch(:slow)})")
+          results.push(result)
+        end
+      end
+
+      new_thread("same_db_parallel_thread2") do
+        ActiveRecord::Base.on_primary do
+          pause_and_mark_ready
+          result = execute_sql("SELECT 'faster query',SLEEP(#{query_delay.fetch(:fast)})")
+          results.push(result)
+        end
+      end
+
+      new_thread("same_db_parallel_thread3") do
+        ActiveRecord::Base.on_primary do
+          pause_and_mark_ready
+          result = execute_sql("SELECT 'medium query',SLEEP(#{query_delay.fetch(:medium)})")
+          results.push(result)
+        end
+      end
+
+      wait_for_threads_to_be_ready
+
+      thread_exection_time = Benchmark.realtime do
+        execute_and_wait_for_threads
+      end
+
+      minimum_serial_query_exection_time = query_delay.values.map(&:to_f).sum
+      # Arbitrarily faster time such that there must have been some parallelization
+      max_parallel_time = minimum_serial_query_exection_time - 0.1
+      assert_operator(max_parallel_time, :>, thread_exection_time)
+
+      rows = results.map(&:first)
+      result_strings = rows.map(&:first)
+      # This order cannot be guaranteed but it likely given the artificial delays
+      assert_equal(
+        [
+          "faster query",
+          "medium query",
+          "slower query"
+        ],
+        result_strings
+      )
+    end
+  end
+
+  def new_thread(name)
+    thread = Thread.new do
+      Thread.current.name = name
+      yield
+    end
+
+    @test_threads ||= []
+    @test_threads.push(thread)
+  end
+
+  def switch_through_all_databases
+    ActiveRecord::Base.on_primary do
+      result = ActiveRecord::Base.connection.execute("SELECT * from accounts")
+      assert_equal("Primary account", record_name(result))
+    end
+    ActiveRecord::Base.on_replica do
+      result = ActiveRecord::Base.connection.execute("SELECT * from accounts")
+      assert_equal("Replica account", record_name(result))
+    end
+    ActiveRecord::Base.on_shard(0) do
+      result = ActiveRecord::Base.connection.execute("SELECT * from tickets")
+      assert_equal("Shard 0 Primary ticket", record_name(result))
+
+      ActiveRecord::Base.on_replica do
+        result = ActiveRecord::Base.connection.execute("SELECT * from tickets")
+        assert_equal("Shard 0 Replica ticket", record_name(result))
+      end
+    end
+    ActiveRecord::Base.on_shard(1) do
+      result = ActiveRecord::Base.connection.execute("SELECT * from tickets")
+      assert_equal("Shard 1 Primary ticket", record_name(result))
+
+      ActiveRecord::Base.on_replica do
+        result = ActiveRecord::Base.connection.execute("SELECT * from tickets")
+        assert_equal("Shard 1 Replica ticket", record_name(result))
+      end
+    end
+  end
+
+  # This allows us to get all of our threads into a prepared state by pausing
+  # them at a 'ready' point so as there is as little overhead as possible
+  # before the interesting code executes.
+  #
+  # Here we use 'ready' to mean the thread is spawned, has had its names set
+  # and has established a database connection.
+  def pause_and_mark_ready
+    Thread.current[:ready] = true
+    sleep
+  end
+
+  def execute_and_wait_for_threads
+    @test_threads.each { |t| t.wakeup if t.alive? }
+    @test_threads.each(&:join)
+  end
+
+  def wait_for_threads_to_be_ready
+    sleep(0.01) until @test_threads.all? { |t| t[:ready] }
+  end
+
+  def use_same_connection_handler_for_all_theads
+    ActiveRecord::Base.default_connection_handler = ActiveRecord::Base.connection_handler
+  end
+
+  def record_name(db_result)
+    name_column_index = 1
+    db_result.first[name_column_index]
+  end
+
+  def execute_sql(query)
+    ActiveRecord::Base.connection.execute(query)
+  end
+
+  def create_seed_data
+    ActiveRecord::Base.on_primary_db do
+      Account.connection.execute(account_insert_sql(name: "Primary account"))
+
+      Account.on_replica do
+        Account.connection.execute(account_insert_sql(name: "Replica account"))
+      end
+    end
+
+    [0, 1].each do |shard_id|
+      ActiveRecord::Base.on_shard(shard_id) do
+        Ticket.connection.execute(ticket_insert_sql(title: "Shard #{shard_id} Primary ticket"))
+
+        Ticket.on_replica do
+          Ticket.connection.execute(ticket_insert_sql(title: "Shard #{shard_id} Replica ticket"))
+        end
+      end
+    end
+  end
+
+  def account_insert_sql(name:)
+    "INSERT INTO accounts (id, name, created_at, updated_at)" \
+      " VALUES (1000, '#{name}', NOW(), NOW())"
+  end
+
+  def ticket_insert_sql(title:)
+    "INSERT INTO tickets (id, title, account_id, created_at, updated_at)" \
+      " VALUES (1000, '#{title}', 5000, NOW(), NOW())"
+  end
+end