diff --git a/CHANGELOG.md b/CHANGELOG.md
index ef600e43..8276edc6 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -14,6 +14,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Added `io.copy_dir()` function.
 - Added new LR schedulers: `LinearWithWarmup`, `InvSqrtWithWarmup`, `ConstantWithWarmup`, `SequentialScheduler`.
 - Added option to pre-download checkpoint files from remote storage before trying to load a checkpoint.
+- Added a callback for sending Slack notifications.
 
 ### Changed
 
diff --git a/src/olmo_core/internal/common.py b/src/olmo_core/internal/common.py
index 5e0ae19c..1c2d426a 100644
--- a/src/olmo_core/internal/common.py
+++ b/src/olmo_core/internal/common.py
@@ -92,6 +92,7 @@ def build_launch_config(
             BeakerEnvSecret(name="AWS_CREDENTIALS", secret=f"{beaker_user}_AWS_CREDENTIALS"),
             BeakerEnvSecret(name="R2_ENDPOINT_URL", secret="R2_ENDPOINT_URL"),
             BeakerEnvSecret(name="WEKA_ENDPOINT_URL", secret="WEKA_ENDPOINT_URL"),
+            BeakerEnvSecret(name="SLACK_WEBHOOK_URL", secret="SLACK_WEBHOOK_URL"),
         ],
         setup_steps=[
             # Clone repo.
diff --git a/src/olmo_core/internal/experiment.py b/src/olmo_core/internal/experiment.py
index 017b98f5..4d1e9ee7 100644
--- a/src/olmo_core/internal/experiment.py
+++ b/src/olmo_core/internal/experiment.py
@@ -36,6 +36,7 @@
     LMEvaluatorCallbackConfig,
     ProfilerCallback,
     SchedulerCallback,
+    SlackNotifierCallback,
     WandBCallback,
 )
 from olmo_core.utils import get_default_device, prepare_cli_environment, seed_all
@@ -171,6 +172,7 @@ def build_common_components(
             ),
             eval_interval=1000,
         ),
+        "slack_notifier": SlackNotifierCallback(name=run_name, enabled=False),
     }
 
     return CommonComponents(
diff --git a/src/olmo_core/train/callbacks/__init__.py b/src/olmo_core/train/callbacks/__init__.py
index f37129f2..0d7883d2 100644
--- a/src/olmo_core/train/callbacks/__init__.py
+++ b/src/olmo_core/train/callbacks/__init__.py
@@ -21,6 +21,7 @@
 from .profiler import ProfilerCallback
 from .scheduler import SchedulerCallback
 from .sequence_length_scheduler import SequenceLengthSchedulerCallback
+from .slack_notifier import SlackNotificationSetting, SlackNotifierCallback
 from .speed_monitor import SpeedMonitorCallback
 from .wandb import WandBCallback
 
@@ -43,6 +44,8 @@
     "GradClipperCallback",
     "MatrixNormalizerCallback",
     "ProfilerCallback",
+    "SlackNotifierCallback",
+    "SlackNotificationSetting",
     "SchedulerCallback",
     "SequenceLengthSchedulerCallback",
     "SpeedMonitorCallback",
diff --git a/src/olmo_core/train/callbacks/slack_notifier.py b/src/olmo_core/train/callbacks/slack_notifier.py
new file mode 100644
index 00000000..57c42d56
--- /dev/null
+++ b/src/olmo_core/train/callbacks/slack_notifier.py
@@ -0,0 +1,117 @@
+import os
+from dataclasses import dataclass
+from typing import Optional
+
+import requests
+
+from olmo_core.config import StrEnum
+from olmo_core.distributed.utils import get_rank
+from olmo_core.exceptions import OLMoEnvironmentError
+
+from .callback import Callback
+
+SLACK_WEBHOOK_URL_ENV_VAR = "SLACK_WEBHOOK_URL"
+
+
+class SlackNotificationSetting(StrEnum):
+    """
+    Defines the notifications settings for the Slack notifier callback.
+    """
+
+    all = "all"
+    """
+    Send all types notifications.
+    """
+
+    end_only = "end_only"
+    """
+    Only send a notification when the experiment ends (successfully or with a failure).
+    """
+
+    failure_only = "failure_only"
+    """
+    Only send a notification when the experiment fails.
+    """
+
+    none = "none"
+    """
+    Don't send any notifcations.
+    """
+
+
+@dataclass
+class SlackNotifierCallback(Callback):
+    name: Optional[str] = None
+    """
+    A name to give the run.
+    """
+
+    notifications: SlackNotificationSetting = SlackNotificationSetting.end_only
+    """
+    The notification settings.
+    """
+
+    enabled: bool = True
+    """
+    Set to false to disable this callback.
+    """
+
+    webhook_url: Optional[str] = None
+    """
+    The webhook URL to post. If not set, will check the environment variable ``SLACK_WEBHOOK_URL``.
+    """
+
+    def post_attach(self):
+        if not self.enabled or get_rank() != 0:
+            return
+
+        if self.webhook_url is None and SLACK_WEBHOOK_URL_ENV_VAR not in os.environ:
+            raise OLMoEnvironmentError(f"missing env var '{SLACK_WEBHOOK_URL_ENV_VAR}'")
+
+    def pre_train(self):
+        if not self.enabled or get_rank() != 0:
+            return
+
+        if self.notifications == SlackNotificationSetting.all:
+            self._post_message("started")
+
+    def post_train(self):
+        if not self.enabled or get_rank() != 0:
+            return
+
+        if self.notifications in (
+            SlackNotificationSetting.all,
+            SlackNotificationSetting.end_only,
+        ):
+            if self.trainer.is_canceled:
+                self._post_message("canceled")
+            else:
+                self._post_message("completed successfully")
+
+    def on_error(self, exc: BaseException):
+        if not self.enabled or get_rank() != 0:
+            return
+
+        if self.notifications in (
+            SlackNotificationSetting.all,
+            SlackNotificationSetting.end_only,
+            SlackNotificationSetting.failure_only,
+        ):
+            self._post_message(f"failed with error:\n{exc}")
+
+    def _post_message(self, msg: str):
+        webhook_url = self.webhook_url or os.environ.get(SLACK_WEBHOOK_URL_ENV_VAR)
+        if webhook_url is None:
+            raise OLMoEnvironmentError(f"missing env var '{SLACK_WEBHOOK_URL_ENV_VAR}'")
+
+        progress = (
+            f"*Progress:*\n"
+            f"- step: {self.step:,d}\n"
+            f"- epoch: {self.trainer.epoch}\n"
+            f"- tokens: {self.trainer.global_train_tokens_seen:,d}"
+        )
+        if self.name is not None:
+            msg = f"Run `{self.name}` {msg}\n{progress}"
+        else:
+            msg = f"Run {msg}\n{progress}"
+        requests.post(webhook_url, json={"text": msg})