diff --git a/.env.example b/.env.example
index 8bf1758a..ebc70610 100644
--- a/.env.example
+++ b/.env.example
@@ -8,6 +8,9 @@ AUTH0_AUDIENCE=https://audience.com/api
 JWT_SECRET_KEY=secret-key
 # Note the list syntax pydantic-settings uses
 ADMIN_ROLES='["Admin", "GalaxyAdmin"]'
+# URL of Galaxy instance, for making calls to Galaxy API
+GALAXY_URL=https://galaxy.example.com
+GALAXY_API_KEY=api-key
 # Comma-separated list of allowed origins. Note we
 #   don't process this with pydantic-settings as it needs
 #   to be used before the FastAPI app loads
diff --git a/auth/config.py b/auth/config.py
index 26f6e50b..2d0212a9 100644
--- a/auth/config.py
+++ b/auth/config.py
@@ -36,7 +36,7 @@ class Settings(BaseSettings):
         "bpa-wheat-pathogens-transcript": "Wheat Pathogens Transcript",
     }
 
-    model_config = SettingsConfigDict(env_file=".env")
+    model_config = SettingsConfigDict(env_file=".env", extra="ignore")
 
 
 @lru_cache()
diff --git a/galaxy/__init__.py b/galaxy/__init__.py
new file mode 100644
index 00000000..e69de29b
diff --git a/galaxy/client.py b/galaxy/client.py
new file mode 100644
index 00000000..e4f2bba1
--- /dev/null
+++ b/galaxy/client.py
@@ -0,0 +1,40 @@
+from typing import Annotated
+
+from fastapi import Depends
+from httpx import Client
+
+from galaxy.config import GalaxySettings, get_galaxy_settings
+from galaxy.schemas import GalaxyUserModel
+
+
+class GalaxyClient:
+    """
+    Client for making requests to the Galaxy API.
+    """
+
+    def __init__(self, galaxy_url: str, api_key: str):
+        """
+        :param galaxy_url: Base URL of the Galaxy instance.
+        :param api_key: API key for a galaxy user.
+        """
+        self.url = galaxy_url
+        self.api_key = api_key
+        self.client = Client(base_url=self.url, headers={'x-api-key': self.api_key})
+
+    def username_exists(self, username: str) -> bool:
+        """
+        Check if a username already exists in Galaxy.
+        Note the user search in the current Galaxy API will return
+        partial matches, so we need to check the returned users
+        for an exact match.
+        """
+        resp = self.client.get("/api/users", params={"f_name": username})
+        returned_users = [GalaxyUserModel(**u) for u in resp.json()]
+        for user in returned_users:
+            if user.username == username:
+                return True
+        return False
+
+
+def get_galaxy_client(settings: Annotated[GalaxySettings, Depends(get_galaxy_settings)]):
+    return GalaxyClient(galaxy_url=settings.galaxy_url, api_key=settings.galaxy_api_key)
diff --git a/galaxy/config.py b/galaxy/config.py
new file mode 100644
index 00000000..3896e642
--- /dev/null
+++ b/galaxy/config.py
@@ -0,0 +1,21 @@
+from functools import lru_cache
+
+from pydantic_settings import BaseSettings, SettingsConfigDict
+
+
+class GalaxySettings(BaseSettings):
+    """
+    Settings for the Galaxy API.
+
+    Note: these are currently read from the same .env file
+    as other settings (which pydantic-settings supports).
+    """
+    galaxy_url: str
+    galaxy_api_key: str
+
+    model_config = SettingsConfigDict(env_file=".env", extra="ignore")
+
+
+@lru_cache
+def get_galaxy_settings():
+    return GalaxySettings()
diff --git a/galaxy/schemas.py b/galaxy/schemas.py
new file mode 100644
index 00000000..4bf0663e
--- /dev/null
+++ b/galaxy/schemas.py
@@ -0,0 +1,16 @@
+from typing import Optional
+
+from pydantic import BaseModel, NaiveDatetime
+
+
+class GalaxyUserModel(BaseModel):
+    """
+    User data returned by Galaxy's users API
+    """
+    model_class: str
+    id: str
+    username: str
+    email: str
+    deleted: bool
+    active: bool
+    last_password_change: Optional[NaiveDatetime]
diff --git a/routers/galaxy_register.py b/routers/galaxy_register.py
index fdea9ddd..926f9481 100644
--- a/routers/galaxy_register.py
+++ b/routers/galaxy_register.py
@@ -1,5 +1,5 @@
 import logging
-from typing import Optional
+from typing import Annotated, Optional
 
 import httpx
 from fastapi import APIRouter, Header, HTTPException
@@ -7,6 +7,7 @@
 
 from auth.config import Settings, get_settings
 from auth.management import get_management_token
+from galaxy.client import GalaxyClient, get_galaxy_client
 from register.tokens import create_registration_token, verify_registration_token
 from schemas.biocommons import BiocommonsRegisterData
 from schemas.galaxy import GalaxyRegistrationData
@@ -26,8 +27,9 @@ async def get_registration_token(settings: Settings = Depends(get_settings)):
 @router.post("/register")
 def register(
         registration_data: GalaxyRegistrationData,
+        settings: Annotated[Settings, Depends(get_settings)],
+        galaxy_client: Annotated[GalaxyClient, Depends(get_galaxy_client)],
         registration_token: Optional[str] = Header(None),
-        settings: Settings = Depends(get_settings),
 ):
     if not registration_token:
         raise HTTPException(status_code=400, detail="Missing registration token")
@@ -35,11 +37,20 @@ def register(
     verify_registration_token(registration_token, settings=settings)
     logger.debug("Registration token verified.")
 
+    user_data = BiocommonsRegisterData.from_galaxy_registration(registration_data)
+    logger.debug("Checking if username exists in Galaxy")
+    galaxy_username = user_data.user_metadata.galaxy_username
+    try:
+        existing = galaxy_client.username_exists(galaxy_username)
+        if existing:
+            raise HTTPException(status_code=400, detail="Username already exists")
+    except httpx.HTTPError as e:
+        logger.warning(f"Failed to check username in Galaxy: {e}")
+
     url = f"https://{settings.auth0_domain}/api/v2/users"
     logger.debug("Getting management token.")
     management_token = get_management_token(settings=settings)
     headers = {"Authorization": f"Bearer {management_token}"}
-    user_data = BiocommonsRegisterData.from_galaxy_registration(registration_data)
     logger.debug("Registering with Auth0 management API")
     resp = httpx.post(
         url,
diff --git a/tests/conftest.py b/tests/conftest.py
index 077fe7d9..5fc716ca 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -1,9 +1,13 @@
+from unittest.mock import MagicMock
+
 import pytest
 from fastapi.testclient import TestClient
 
 from auth.config import Settings, get_settings
 from auth.management import get_management_token
 from auth.validator import get_current_user
+from galaxy.client import GalaxyClient, get_galaxy_client
+from galaxy.config import GalaxySettings, get_galaxy_settings
 from main import app
 from tests.datagen import AccessTokenPayloadFactory, SessionUserFactory
 
@@ -16,7 +20,10 @@ def ignore_env_file():
     """
     def get_settings_no_env_file():
         return Settings(_env_file=None)
+    def get_galaxy_settings_no_env_file():
+        return GalaxySettings(_env_file=None)
     app.dependency_overrides[get_settings] = get_settings_no_env_file
+    app.dependency_overrides[get_galaxy_settings] = get_galaxy_settings_no_env_file
 
 
 @pytest.fixture
@@ -33,8 +40,15 @@ def mock_settings():
         auth0_algorithms=["HS256"]
     )
 
+
+@pytest.fixture
+def mock_galaxy_settings():
+    """Dummy settings values for GalaxySettings"""
+    return GalaxySettings(galaxy_url="http://mock-url", galaxy_api_key="mock-key")
+
+
 @pytest.fixture
-def test_client(mock_settings):
+def test_client(mock_settings, mock_galaxy_settings):
     """
     Override the get_settings dependency to return a mocked Settings object.
     """
@@ -44,6 +58,7 @@ def override_settings():
 
     # Apply override
     app.dependency_overrides[get_settings] = override_settings
+    app.dependency_overrides[get_galaxy_settings] = lambda: mock_galaxy_settings
 
     # Create client
     client = TestClient(app)
@@ -67,3 +82,11 @@ def override_user():
     app.dependency_overrides[get_management_token] = lambda: "mock_token"
     yield
     app.dependency_overrides.clear()
+
+
+@pytest.fixture
+def mock_galaxy_client():
+    client = MagicMock(GalaxyClient)
+    app.dependency_overrides[get_galaxy_client] = lambda: client
+    yield client
+    app.dependency_overrides.clear()
diff --git a/tests/galaxy/__init__.py b/tests/galaxy/__init__.py
new file mode 100644
index 00000000..e69de29b
diff --git a/tests/galaxy/data_generation.py b/tests/galaxy/data_generation.py
new file mode 100644
index 00000000..9c3f6527
--- /dev/null
+++ b/tests/galaxy/data_generation.py
@@ -0,0 +1,6 @@
+from polyfactory.factories.pydantic_factory import ModelFactory
+
+from galaxy.schemas import GalaxyUserModel
+
+
+class GalaxyUserFactory(ModelFactory[GalaxyUserModel]): ...
diff --git a/tests/galaxy/test_client.py b/tests/galaxy/test_client.py
new file mode 100644
index 00000000..9439c4d9
--- /dev/null
+++ b/tests/galaxy/test_client.py
@@ -0,0 +1,25 @@
+import httpx
+import pytest
+
+from galaxy.client import GalaxyClient
+from tests.galaxy.data_generation import GalaxyUserFactory
+
+
+@pytest.fixture
+def galaxy_client():
+    return GalaxyClient(galaxy_url="https://galaxy.example.com",
+                        api_key="dummy-key")
+
+
+def test_username_exists(galaxy_client, respx_mock):
+    user1 = GalaxyUserFactory.build(username="user1")
+    user2 = GalaxyUserFactory.build(username="user2")
+    respx_mock.get("https://galaxy.example.com/api/users").mock(
+        return_value=httpx.Response(
+            200,
+            json=[user1.model_dump(mode="json"),
+                  user2.model_dump(mode="json")]
+        )
+    )
+    assert galaxy_client.username_exists("user1")
+    assert not galaxy_client.username_exists("other_user")
diff --git a/tests/test_galaxy.py b/tests/test_galaxy.py
index a04012ef..24c0435f 100644
--- a/tests/test_galaxy.py
+++ b/tests/test_galaxy.py
@@ -138,7 +138,7 @@ def test_register(mocker, mock_auth_token, mock_settings, test_client):
 
 
 @pytest.mark.respx(base_url="https://mock-domain")
-def test_register_json_types(respx_mock, mock_auth_token, mock_settings, test_client):
+def test_register_json_types(respx_mock, mock_auth_token, mock_settings, test_client, mock_galaxy_client):
     """
     Test how we handle datetimes in the response data: if we don't
     use model_dump(mode="json") when providing json data, we can get errors
@@ -154,6 +154,7 @@ def test_register_json_types(respx_mock, mock_auth_token, mock_settings, test_cl
     user_data = GalaxyRegistrationDataFactory.build()
     token_resp = test_client.get("/galaxy/get-registration-token")
     headers = {"registration-token": token_resp.json()["token"]}
+    mock_galaxy_client.username_exists.return_value = False
     resp = test_client.post("/galaxy/register", json=user_data.model_dump(), headers=headers)
     assert resp.status_code == 200