diff --git a/docs/python-api.md b/docs/python-api.md
index c81df56..bee4785 100644
--- a/docs/python-api.md
+++ b/docs/python-api.md
@@ -96,3 +96,20 @@ print(response2.text())
 You will get back five fun facts about skunks.
 
 Access `conversation.responses` for a list of all of the responses that have so far been returned during the conversation.
+
+## Other functions
+
+The `llm` top level package includes some useful utility functions.
+
+### set_alias(alias, model_id)
+
+The `llm.set_alias()` function can be used to define a new alias:
+
+```python
+import llm
+
+llm.set_alias("turbo", "gpt-3.5-turbo")
+```
+The second argument can be a model identifier or another alias, in which case that alias will be resolved.
+
+If the `aliases.json` file does not exist or contains invalid JSON it will be created or overwritten.
diff --git a/llm/__init__.py b/llm/__init__.py
index 190b9d8..1cbbfe9 100644
--- a/llm/__init__.py
+++ b/llm/__init__.py
@@ -131,3 +131,27 @@ def user_dir():
     if llm_user_path:
         return pathlib.Path(llm_user_path)
     return pathlib.Path(click.get_app_dir("io.datasette.llm"))
+
+
+def set_alias(alias, model_id_or_alias):
+    """
+    Set an alias to point to the specified model.
+    """
+    path = user_dir() / "aliases.json"
+    path.parent.mkdir(parents=True, exist_ok=True)
+    if not path.exists():
+        path.write_text("{}\n")
+    try:
+        current = json.loads(path.read_text())
+    except json.decoder.JSONDecodeError:
+        # We're going to write a valid JSON file in a moment:
+        current = {}
+    # Resolve model_id_or_alias to a model_id
+    try:
+        model = get_model(model_id_or_alias)
+        model_id = model.model_id
+    except UnknownModelError:
+        # Set the alias to the exact string they provided instead
+        model_id = model_id_or_alias
+    current[alias] = model_id
+    path.write_text(json.dumps(current, indent=4) + "\n")
diff --git a/tests/test_aliases.py b/tests/test_aliases.py
index 6c256b5..8fc2d8d 100644
--- a/tests/test_aliases.py
+++ b/tests/test_aliases.py
@@ -1,11 +1,19 @@
 from click.testing import CliRunner
 from llm.cli import cli
+import llm
 import json
 import pytest
 
 
+def test_set_alias():
+    with pytest.raises(llm.UnknownModelError):
+        llm.get_model("this-is-a-new-alias")
+    llm.set_alias("this-is-a-new-alias", "gpt-3.5-turbo")
+    assert llm.get_model("this-is-a-new-alias").model_id == "gpt-3.5-turbo"
+
+
 @pytest.mark.parametrize("args", (["aliases", "list"], ["aliases"]))
-def test_aliases_list(args):
+def test_cli_aliases_list(args):
     runner = CliRunner()
     result = runner.invoke(cli, args)
     assert result.exit_code == 0
@@ -21,7 +29,7 @@ def test_aliases_list(args):
 
 
 @pytest.mark.parametrize("args", (["aliases", "list"], ["aliases"]))
-def test_aliases_list_json(args):
+def test_cli_aliases_list_json(args):
     runner = CliRunner()
     result = runner.invoke(cli, args + ["--json"])
     assert result.exit_code == 0
@@ -36,7 +44,7 @@ def test_aliases_list_json(args):
     }
 
 
-def test_aliases_set(user_path):
+def test_cli_aliases_set(user_path):
     # Should be not aliases.json at start
     assert not (user_path / "aliases.json").exists()
     runner = CliRunner()
@@ -46,14 +54,14 @@ def test_aliases_set(user_path):
     assert json.loads((user_path / "aliases.json").read_text("utf-8")) == {"foo": "bar"}
 
 
-def test_aliases_path(user_path):
+def test_cli_aliases_path(user_path):
     runner = CliRunner()
     result = runner.invoke(cli, ["aliases", "path"])
     assert result.exit_code == 0
     assert result.output.strip() == str(user_path / "aliases.json")
 
 
-def test_aliases_remove(user_path):
+def test_cli_aliases_remove(user_path):
     (user_path / "aliases.json").write_text(json.dumps({"foo": "bar"}), "utf-8")
     runner = CliRunner()
     result = runner.invoke(cli, ["aliases", "remove", "foo"])
@@ -61,7 +69,7 @@ def test_aliases_remove(user_path):
     assert json.loads((user_path / "aliases.json").read_text("utf-8")) == {}
 
 
-def test_aliases_remove_invalid(user_path):
+def test_cli_aliases_remove_invalid(user_path):
     (user_path / "aliases.json").write_text(json.dumps({"foo": "bar"}), "utf-8")
     runner = CliRunner()
     result = runner.invoke(cli, ["aliases", "remove", "invalid"])
@@ -70,7 +78,7 @@ def test_aliases_remove_invalid(user_path):
 
 
 @pytest.mark.parametrize("args", (["models"], ["models", "list"]))
-def test_aliases_are_registered(user_path, args):
+def test_cli_aliases_are_registered(user_path, args):
     (user_path / "aliases.json").write_text(
         json.dumps({"foo": "bar", "turbo": "gpt-3.5-turbo"}), "utf-8"
     )