diff --git a/toot/cli/__init__.py b/toot/cli/__init__.py
index 0c71cb3..1f33d1a 100644
--- a/toot/cli/__init__.py
+++ b/toot/cli/__init__.py
@@ -110,6 +110,20 @@ class AccountParamType(StringParamType):
         ]
 
 
+class InstanceParamType(StringParamType):
+    """Custom type to add shell completion for instance domains"""
+    name = "instance"
+
+    def shell_complete(self, ctx, param, incomplete: str):
+        apps = config.load_config()["apps"]
+
+        return [
+            CompletionItem(i)
+            for i in apps.keys()
+            if i.lower().startswith(incomplete.lower())
+        ]
+
+
 def pass_context(f: "t.Callable[te.Concatenate[Context, P], R]") -> "t.Callable[P, R]":
     """Pass the toot Context as first argument."""
     @wraps(f)
diff --git a/toot/cli/read.py b/toot/cli/read.py
index cfbcea8..32ce49a 100644
--- a/toot/cli/read.py
+++ b/toot/cli/read.py
@@ -9,7 +9,7 @@ from toot.cli.validators import validate_instance
 from toot.entities import Instance, Status, from_dict, Account
 from toot.exceptions import ApiError, ConsoleError
 from toot.output import print_account, print_instance, print_search_results, print_status, print_timeline
-from toot.cli import cli, get_context, json_option, pass_context, Context
+from toot.cli import InstanceParamType, cli, get_context, json_option, pass_context, Context
 
 
 @cli.command()
@@ -43,7 +43,7 @@ def whois(ctx: Context, account: str, json: bool):
 
 
 @cli.command()
-@click.argument("instance", callback=validate_instance, required=False)
+@click.argument("instance", type=InstanceParamType(), callback=validate_instance, required=False)
 @json_option
 def instance(instance: Optional[str], json: bool):
     """Display instance details
diff --git a/toot/cli/timelines.py b/toot/cli/timelines.py
index 86d0ceb..aac2c45 100644
--- a/toot/cli/timelines.py
+++ b/toot/cli/timelines.py
@@ -2,7 +2,7 @@ import sys
 import click
 
 from toot import api
-from toot.cli import cli, get_context, pass_context, Context
+from toot.cli import InstanceParamType, cli, get_context, pass_context, Context
 from typing import Optional
 from toot.cli.validators import validate_instance
 
@@ -13,6 +13,7 @@ from toot.output import print_notifications, print_timeline
 @cli.command()
 @click.option(
     "--instance", "-i",
+    type=InstanceParamType(),
     callback=validate_instance,
     help="""Domain or base URL of the instance from which to read,
             e.g. 'mastodon.social' or 'https://mastodon.social'""",