diff --git a/temporalio/lib/temporalio/activity/context.rb b/temporalio/lib/temporalio/activity/context.rb
index edaef5e3..5957a6e6 100644
--- a/temporalio/lib/temporalio/activity/context.rb
+++ b/temporalio/lib/temporalio/activity/context.rb
@@ -108,9 +108,16 @@ def _scoped_logger_info
       end
 
       # @return [Metric::Meter] Metric meter to create metrics on, with some activity-specific attributes already set.
+      # @raise [RuntimeError] Called within a {Testing::ActivityEnvironment} and it was not set.
       def metric_meter
         raise NotImplementedError
       end
+
+      # @return [Client] Temporal client this activity worker is running in.
+      # @raise [RuntimeError] Called within a {Testing::ActivityEnvironment} and it was not set.
+      def client
+        raise NotImplementedError
+      end
     end
   end
 end
diff --git a/temporalio/lib/temporalio/internal/worker/activity_worker.rb b/temporalio/lib/temporalio/internal/worker/activity_worker.rb
index 595a63e6..3cd7c742 100644
--- a/temporalio/lib/temporalio/internal/worker/activity_worker.rb
+++ b/temporalio/lib/temporalio/internal/worker/activity_worker.rb
@@ -201,6 +201,7 @@ def execute_activity(task_token, defn, start)
 
           # Run
           activity = RunningActivity.new(
+            worker: @worker,
             info:,
             cancellation: Cancellation.new,
             worker_shutdown_cancellation: @worker._worker_shutdown_cancellation,
@@ -299,6 +300,7 @@ class RunningActivity < Activity::Context
           attr_accessor :instance, :_outbound_impl, :_server_requested_cancel
 
           def initialize( # rubocop:disable Lint/MissingSuper
+            worker:,
             info:,
             cancellation:,
             worker_shutdown_cancellation:,
@@ -306,6 +308,7 @@ def initialize( # rubocop:disable Lint/MissingSuper
             logger:,
             runtime_metric_meter:
           )
+            @worker = worker
             @info = info
             @cancellation = cancellation
             @worker_shutdown_cancellation = worker_shutdown_cancellation
@@ -334,6 +337,10 @@ def metric_meter
               }
             )
           end
+
+          def client
+            @worker.client
+          end
         end
 
         class InboundImplementation < Temporalio::Worker::Interceptor::Activity::Inbound
diff --git a/temporalio/lib/temporalio/testing/activity_environment.rb b/temporalio/lib/temporalio/testing/activity_environment.rb
index 6d504ac3..73720364 100644
--- a/temporalio/lib/temporalio/testing/activity_environment.rb
+++ b/temporalio/lib/temporalio/testing/activity_environment.rb
@@ -47,6 +47,9 @@ def self.default_info
       # @param payload_converter [Converters::PayloadConverter] Value for {Activity::Context#payload_converter}.
       # @param logger [Logger] Value for {Activity::Context#logger}.
       # @param activity_executors [Hash<Symbol, Worker::ActivityExecutor>] Executors that activities can run within.
+      # @param metric_meter [Metric::Meter, nil] Value for {Activity::Context#metric_meter}, or nil to raise when
+      #   called.
+      # @param client [Client, nil] Value for {Activity::Context#client}, or nil to raise when called.
       def initialize(
         info: ActivityEnvironment.default_info,
         on_heartbeat: nil,
@@ -54,7 +57,9 @@ def initialize(
         worker_shutdown_cancellation: Cancellation.new,
         payload_converter: Converters::PayloadConverter.default,
         logger: Logger.new(nil),
-        activity_executors: Worker::ActivityExecutor.defaults
+        activity_executors: Worker::ActivityExecutor.defaults,
+        metric_meter: nil,
+        client: nil
       )
         @info = info
         @on_heartbeat = on_heartbeat
@@ -63,6 +68,8 @@ def initialize(
         @payload_converter = payload_converter
         @logger = logger
         @activity_executors = activity_executors
+        @metric_meter = metric_meter
+        @client = client
       end
 
       # Run an activity and returns its result or raises its exception.
@@ -86,7 +93,9 @@ def run(activity, *args)
                                                 cancellation: @cancellation,
                                                 worker_shutdown_cancellation: @worker_shutdown_cancellation,
                                                 payload_converter: @payload_converter,
-                                                logger: @logger
+                                                logger: @logger,
+                                                metric_meter: @metric_meter,
+                                                client: @client
                                               ))
           queue.push([defn.proc.call(*args), nil])
         rescue Exception => e # rubocop:disable Lint/RescueException Intentionally capturing all exceptions
@@ -113,7 +122,9 @@ def initialize( # rubocop:disable Lint/MissingSuper
           cancellation:,
           worker_shutdown_cancellation:,
           payload_converter:,
-          logger:
+          logger:,
+          metric_meter:,
+          client:
         )
           @info = info
           @instance = instance
@@ -122,12 +133,24 @@ def initialize( # rubocop:disable Lint/MissingSuper
           @worker_shutdown_cancellation = worker_shutdown_cancellation
           @payload_converter = payload_converter
           @logger = logger
+          @metric_meter = metric_meter
+          @client = client
         end
 
         # @!visibility private
         def heartbeat(*details)
           @on_heartbeat&.call(details)
         end
+
+        # @!visibility private
+        def metric_meter
+          @metric_meter or raise 'No metric meter configured in this test environment'
+        end
+
+        # @!visibility private
+        def client
+          @client or raise 'No client configured in this test environment'
+        end
       end
 
       private_constant :Context
diff --git a/temporalio/lib/temporalio/worker.rb b/temporalio/lib/temporalio/worker.rb
index 4a0cb216..846e8093 100644
--- a/temporalio/lib/temporalio/worker.rb
+++ b/temporalio/lib/temporalio/worker.rb
@@ -55,6 +55,8 @@ class Worker
     )
 
     # Options as returned from {options} for `**to_h` splat use in {initialize}. See {initialize} for details.
+    #
+    # Note, the `client` within can be replaced via client setter.
     class Options; end # rubocop:disable Lint/EmptyClass
 
     # @return [String] Memoized default build ID. This default value is built as a checksum of all of the loaded Ruby
@@ -484,6 +486,9 @@ def initialize(
 
       # Validate worker
       @bridge_worker.validate
+
+      # Mutex needed for accessing and replacing a client
+      @client_mutex = Mutex.new
     end
 
     # @return [String] Task queue set on the worker options.
@@ -491,6 +496,25 @@ def task_queue
       @options.task_queue
     end
 
+    # @return [Client] Client for this worker. This is the same as {Options.client} in {options}, but surrounded by a
+    #   mutex to be safe for client replacement in {client=}.
+    def client
+      @client_mutex.synchronize { @options.client }
+    end
+
+    # Replace the worker's client. When this is called, the client is replaced on the internal worker which means any
+    # new calls will be made on the new client (but existing calls will still complete on the previous one). This is
+    # commonly used for providing a new client with updated authentication credentials.
+    #
+    # @param new_client [Client] New client to use for new calls.
+    def client=(new_client)
+      @client_mutex.synchronize do
+        @bridge_worker.replace_client(new_client.connection._core_client)
+        @options = @options.with(client: new_client)
+        new_client
+      end
+    end
+
     # Run this worker until cancellation or optional block completes. When the cancellation or block is complete, the
     # worker is shut down. This will return the block result if everything successful or raise an error if not.
     #
diff --git a/temporalio/sig/temporalio/activity/context.rbs b/temporalio/sig/temporalio/activity/context.rbs
index 930522b4..2aefb2b1 100644
--- a/temporalio/sig/temporalio/activity/context.rbs
+++ b/temporalio/sig/temporalio/activity/context.rbs
@@ -19,6 +19,7 @@ module Temporalio
       def _scoped_logger_info: -> Hash[Symbol, Object]
 
       def metric_meter: -> Metric::Meter
+      def client: -> Client
     end
   end
 end
\ No newline at end of file
diff --git a/temporalio/sig/temporalio/internal/worker/activity_worker.rbs b/temporalio/sig/temporalio/internal/worker/activity_worker.rbs
index 651f3caf..16709839 100644
--- a/temporalio/sig/temporalio/internal/worker/activity_worker.rbs
+++ b/temporalio/sig/temporalio/internal/worker/activity_worker.rbs
@@ -32,6 +32,7 @@ module Temporalio
           attr_accessor _server_requested_cancel: bool
 
           def initialize: (
+            worker: Temporalio::Worker,
             info: Activity::Info,
             cancellation: Cancellation,
             worker_shutdown_cancellation: Cancellation,
diff --git a/temporalio/sig/temporalio/testing/activity_environment.rbs b/temporalio/sig/temporalio/testing/activity_environment.rbs
index 5480c6df..e2379b24 100644
--- a/temporalio/sig/temporalio/testing/activity_environment.rbs
+++ b/temporalio/sig/temporalio/testing/activity_environment.rbs
@@ -10,7 +10,9 @@ module Temporalio
         ?worker_shutdown_cancellation: Cancellation,
         ?payload_converter: Converters::PayloadConverter,
         ?logger: Logger,
-        ?activity_executors: Hash[Symbol, Worker::ActivityExecutor]
+        ?activity_executors: Hash[Symbol, Worker::ActivityExecutor],
+        ?metric_meter: Metric::Meter?,
+        ?client: Client?
       ) -> void
 
       def run: (
diff --git a/temporalio/sig/temporalio/worker.rbs b/temporalio/sig/temporalio/worker.rbs
index 9da961c7..f5c3e465 100644
--- a/temporalio/sig/temporalio/worker.rbs
+++ b/temporalio/sig/temporalio/worker.rbs
@@ -60,6 +60,8 @@ module Temporalio
         workflow_payload_codec_thread_pool: ThreadPool?,
         debug_mode: bool
       ) -> void
+
+      def with: (**Object kwargs) -> Options
     end
 
     def self.default_build_id: -> String
@@ -110,6 +112,9 @@ module Temporalio
 
     def task_queue: -> String
 
+    def client: -> Client
+    def client=: (Client new_client) -> void
+
     def run: [T] (
       ?cancellation: Cancellation,
       ?shutdown_signals: Array[String | Integer],
diff --git a/temporalio/test/worker_activity_test.rb b/temporalio/test/worker_activity_test.rb
index 8c870066..a95c0e6b 100644
--- a/temporalio/test/worker_activity_test.rb
+++ b/temporalio/test/worker_activity_test.rb
@@ -901,6 +901,19 @@ def test_context_instance
                  execute_activity(shared_instance, interceptors: [ContextInstanceInterceptor.new])
   end
 
+  class ClientAccessActivity < Temporalio::Activity::Definition
+    def execute
+      desc = Temporalio::Activity::Context.current.client.workflow_handle(
+        Temporalio::Activity::Context.current.info.workflow_id
+      ).describe
+      desc.raw_description.pending_activities.first.activity_type.name
+    end
+  end
+
+  def test_client_access
+    assert_equal 'ClientAccessActivity', execute_activity(ClientAccessActivity)
+  end
+
   # steep:ignore
   def execute_activity(
     activity,
diff --git a/temporalio/test/worker_workflow_test.rb b/temporalio/test/worker_workflow_test.rb
index 50a5100f..57b71268 100644
--- a/temporalio/test/worker_workflow_test.rb
+++ b/temporalio/test/worker_workflow_test.rb
@@ -1769,13 +1769,59 @@ def test_context_instance
                  execute_workflow(ContextInstanceWorkflow, interceptors: [ContextInstanceInterceptor.new])
   end
 
+  class WorkerClientReplacementWorkflow < Temporalio::Workflow::Definition
+    def execute
+      Temporalio::Workflow.wait_condition { @complete }
+    end
+
+    workflow_signal
+    def complete(value)
+      @complete = value
+    end
+  end
+
+  def test_worker_client_replacement
+    # Create a second ephemeral server and start workflow on both servers
+    Temporalio::Testing::WorkflowEnvironment.start_local do |env2|
+      # Start both workflows on different servers
+      task_queue = "tq-#{SecureRandom.uuid}"
+      handle1 = env.client.start_workflow(WorkerClientReplacementWorkflow, id: "wf-#{SecureRandom.uuid}", task_queue:)
+      handle2 = env2.client.start_workflow(WorkerClientReplacementWorkflow, id: "wf-#{SecureRandom.uuid}", task_queue:)
+
+      # Run worker on the first env. Make sure cache is off and only 1 max poller
+      worker = Temporalio::Worker.new(
+        client: env.client, task_queue:, workflows: [WorkerClientReplacementWorkflow],
+        max_cached_workflows: 0, max_concurrent_workflow_task_polls: 1
+      )
+      worker.run do
+        # Confirm first workflow has a task complete but not the second
+        assert_eventually do
+          refute_nil handle1.fetch_history_events.find(&:workflow_task_completed_event_attributes)
+        end
+        assert_nil handle2.fetch_history_events.find(&:workflow_task_completed_event_attributes)
+
+        # Replace the client
+        worker.client = env2.client
+
+        # Signal both which should allow the current poll to wake up and it'll be a task failure when trying to submit
+        # that to the new client which is ignored. But also the new client will poll for the new workflow, which we will
+        # wait for it to complete.
+        handle1.signal(WorkerClientReplacementWorkflow.complete, 'done1')
+        handle2.signal(WorkerClientReplacementWorkflow.complete, 'done2')
+
+        # Confirm second workflow on new server completes
+        assert_equal 'done2', handle2.result
+        handle1.terminate
+      end
+    end
+  end
+
   # TODO(cretz): To test
   # * Common
   #   * Eager workflow start
   #   * Unawaited futures that have exceptions, need to log warning like Java does
   #   * Enhanced stack trace?
   #   * Separate abstract/interface demonstration
-  #   * Replace worker client
   #   * Reset update randomness seed
   #   * Confirm thread pool does not leak, meaning thread/worker goes away after last workflow
   #   * Test workflow cancel causing other cancels at the same time but in different coroutines