diff --git a/RELEASE.rst b/RELEASE.rst
new file mode 100644
index 0000000..be7273f
--- /dev/null
+++ b/RELEASE.rst
@@ -0,0 +1,3 @@
+type: minor
+---
+Add support for uploading to the steam workshop.
diff --git a/mypy.ini b/mypy.ini
index d4f9ca9..0ef4f55 100644
--- a/mypy.ini
+++ b/mypy.ini
@@ -2,3 +2,6 @@
 mypy_path = stubs
 files = src
 strict = True
+
+[mypy-steam.*]
+ignore_missing_imports = True
diff --git a/pyproject.toml b/pyproject.toml
index 758995a..6078b93 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -6,6 +6,7 @@ build-backend = "flit_core.buildapi"
 module = "tts"
 dist-name = "tabletop-tools"
 requires = [
+    "appdirs",
     "attrs",
     "bson",
     "requests",
@@ -16,6 +17,11 @@ author-email = "tom.prince@hocat.ca"
 description-file = "README.rst"
 classifiers = ["License :: OSI Approved :: Mozilla Public License 2.0 (MPL 2.0)"]
 
+[tool.flit.metadata.requires-extra]
+upload = [
+    "steam",
+]
+
 [tool.flit.metadata.urls]
 Repository = "https://github.com/tomprince/tabletop-tools"
 
@@ -26,3 +32,6 @@ tts = "tts.cli:main"
 [tool.tts_tooling]
 release_file = "RELEASE.rst"
 changelog = "CHANGELOG.rst"
+
+[tool.black]
+include = '\.pyi?$'
diff --git a/src/tts/cli.py b/src/tts/cli.py
index 2b15d7b..c6d7569 100644
--- a/src/tts/cli.py
+++ b/src/tts/cli.py
@@ -36,18 +36,47 @@ def unpack_cmd(*, savegame_file: Optional[Path], fileid: Optional[int]) -> None:
     dest="savegame_file",
     type=Path,
     nargs="?",
-    default="build/savegame.json",
 )
-def repack_cmd(*, savegame_file: Path) -> None:
+@app.argument("--fileid", type=int, help="Workshop file id to unpack.")
+@app.argument("--binary", action="store_true")
+def repack_cmd(
+    *, savegame_file: Optional[Path], fileid: Optional[int], binary: bool
+) -> None:
     from .config import config
     from .repack import repack
 
-    if not savegame_file.parent.exists():
+    if fileid and savegame_file:
+        raise Exception("Can't specify both a savegame file and a workshop file id.")
+    elif fileid and binary:
+        raise Exception("Can't specify both a workshop file id and '--binary'.")
+    elif not savegame_file:
+        if binary:
+            savegame_file = Path("build/savegame.bson")
+        else:
+            savegame_file = Path("build/savegame.json")
+
+    if not fileid and not savegame_file.parent.exists():
         savegame_file.parent.mkdir(parents=True)
 
     savegame = repack(config=config)
 
-    savegame_file.write_text(format_json(savegame))
+    if fileid:
+        import bson
+
+        from tts.steam import cli_login, update_file, upload_file
+
+        client = cli_login()
+        # It appears that tabletop simulator depends on the file being named
+        # `WorkshopUpload`.
+        upload_file(client, "WorkshopUpload", bson.dumps(savegame))
+        update_file(client, fileid, "WorkshopUpload")
+
+    elif binary:
+        import bson
+
+        savegame_file.write_bytes(bson.dumps(savegame))
+    else:
+        savegame_file.write_text(format_json(savegame))
 
 
 @app.command("workshop-download", help="Download a mod from the steam workshop.")
@@ -65,4 +94,28 @@ def download_cmd(*, fileid: int, output: Optional[Path]) -> None:
     output.write_text(format_json(mod))
 
 
+@app.command(
+    "workshop-upload",
+    help="Upload a mod to the steam workshop.",
+    description="This will currently only update a existing mod.",
+)
+@app.argument("fileid", type=int)
+@app.argument(
+    "savegame_file",
+    metavar="savegame",
+    type=Path,
+)
+def upload_cmd(*, fileid: int, savegame_file: Path) -> None:
+    import bson
+
+    from tts.steam import cli_login, update_file, upload_file
+
+    savegame = json.loads(savegame_file.read_text())
+    client = cli_login()
+    # It appears that tabletop simulator depends on the file being named
+    # `WorkshopUpload`.
+    upload_file(client, "WorkshopUpload", bson.dumps(savegame))
+    update_file(client, fileid, "WorkshopUpload")
+
+
 main = app.main
diff --git a/src/tts/config.py b/src/tts/config.py
index c7a40e2..17a19bf 100644
--- a/src/tts/config.py
+++ b/src/tts/config.py
@@ -1,6 +1,11 @@
 from pathlib import Path
 
 import attr
+from appdirs import AppDirs
+
+APPID = 286160
+
+appdirs = AppDirs("tabletop-tools", appauthor=False)
 
 
 @attr.s(auto_attribs=True)
diff --git a/src/tts/steam/__init__.py b/src/tts/steam/__init__.py
new file mode 100644
index 0000000..0666162
--- /dev/null
+++ b/src/tts/steam/__init__.py
@@ -0,0 +1,33 @@
+from pathlib import Path
+
+from steam.client import SteamClient
+from steam.enums.common import EResult
+
+from ..config import APPID, appdirs
+from ._upload import upload_file
+
+__all__ = ["cli_login", "update_file", "upload_file"]
+
+
+def cli_login() -> SteamClient:
+    cache_dir = Path(appdirs.user_cache_dir)
+    client = SteamClient()
+    client.credential_location = cache_dir.joinpath("steam")
+    res = client.cli_login()
+    if res != EResult.OK:
+        raise Exception(f"Could not login: {res.name}")
+
+    return client
+
+
+def update_file(client: SteamClient, file_id: int, file_name: str) -> None:
+    resp = client.send_um_and_wait(
+        "PublishedFile.Update#1",
+        {
+            "appid": APPID,
+            "publishedfileid": file_id,
+            "filename": file_name,
+        },
+    )
+    if resp.header.eresult != EResult.OK:
+        raise Exception(f"Couldn't publish file: {EResult(resp.header.eresult).name}")
diff --git a/src/tts/steam/_upload.py b/src/tts/steam/_upload.py
new file mode 100644
index 0000000..297a615
--- /dev/null
+++ b/src/tts/steam/_upload.py
@@ -0,0 +1,85 @@
+import hashlib
+import time
+
+import requests
+from steam.client import SteamClient
+from steam.enums.common import EResult
+from steam.protobufs.steammessages_cloud_pb2 import ClientCloudFileUploadBlockDetails
+
+from ..config import APPID
+
+
+def _hash_sha1(data: bytes) -> bytes:
+    m = hashlib.sha1()
+    m.update(data)
+    return m.digest()
+
+
+def upload_file(client: SteamClient, file_name: str, file_data: bytes) -> None:
+    file_sha = _hash_sha1(file_data)
+    resp = client.send_um_and_wait(
+        "Cloud.ClientBeginFileUpload#1",
+        {
+            "appid": APPID,
+            "file_size": len(file_data),
+            "raw_file_size": len(file_data),
+            "file_sha": file_sha,
+            "filename": file_name,
+            "can_encrypt": False,
+            "time_stamp": int(time.time()),
+            "is_shared_file": False,
+        },
+    )
+    if resp.header.eresult == EResult.DuplicateRequest:
+        # Already uploaded.
+        return
+    if resp.header.eresult != EResult.OK:
+        raise Exception(f"Couldn't start upload: {EResult(resp.header.eresult).name}")
+    try:
+        for block_request in resp.body.block_requests:
+            _upload_block(block_request, file_data)
+    except Exception:
+        client.send_um_and_wait(
+            "Cloud.ClientCommitFileUpload#1",
+            {
+                "appid": APPID,
+                "filename": file_name,
+                "file_sha": file_sha,
+                "transfer_succeeded": False,
+            },
+        )
+        raise
+    else:
+        resp = client.send_um_and_wait(
+            "Cloud.ClientCommitFileUpload#1",
+            {
+                "appid": APPID,
+                "filename": file_name,
+                "file_sha": file_sha,
+                "transfer_succeeded": True,
+            },
+        )
+        if resp.header.eresult != EResult.OK:
+            raise Exception(
+                f"Couldn't commit upload: {EResult(resp.header.eresult).name}"
+            )
+
+
+def _upload_block(
+    block_request: ClientCloudFileUploadBlockDetails, file_data: bytes
+) -> None:
+    url = f"https://{block_request.url_host}{block_request.url_path}"
+    headers = {}
+    for header in block_request.request_headers:
+        if header.name == "Content-Disposition":
+            headers[header.name] = header.value.rstrip(";")
+        else:
+            headers[header.name] = header.value
+    block_end = block_request.block_offset + block_request.block_length
+    block_data = file_data[block_request.block_offset : block_end]
+    headers["Content-Length"] = f"{block_request.block_length}"
+    headers[
+        "Content-Range"
+    ] = f"bytes {block_request.block_offset}-{block_end}/{len(file_data)}"
+    response = requests.put(url, headers=headers, data=block_data)
+    response.raise_for_status()
diff --git a/stubs/appdirs.pyi b/stubs/appdirs.pyi
new file mode 100644
index 0000000..2a8fb13
--- /dev/null
+++ b/stubs/appdirs.pyi
@@ -0,0 +1,26 @@
+from typing import Literal, Optional, Union
+
+
+class AppDirs:
+    def __init__(
+        self,
+        appname: Optional[str] = ...,
+        appauthor: Union[str, Literal[False], None] = ...,
+        version: Optional[str] = ...,
+        roaming: bool = ...,
+        multipath: bool = ...,
+    ) -> None: ...
+    @property
+    def user_data_dir(self) -> str: ...
+    @property
+    def site_data_dir(self) -> str: ...
+    @property
+    def user_config_dir(self) -> str: ...
+    @property
+    def site_config_dir(self) -> str: ...
+    @property
+    def user_cache_dir(self) -> str: ...
+    @property
+    def user_state_dir(self) -> str: ...
+    @property
+    def user_log_dir(self) -> str: ...
diff --git a/stubs/bson.pyi b/stubs/bson.pyi
index df81dec..8cc10b0 100644
--- a/stubs/bson.pyi
+++ b/stubs/bson.pyi
@@ -1,4 +1,4 @@
 from typing import Any, Dict
 
-def loads(data: bytes) -> Dict[Any, Any]:
-    ...
+def loads(data: bytes) -> Dict[Any, Any]: ...
+def dumps(data: Dict[Any, Any]) -> bytes: ...