diff --git a/swh/model/hypothesis_strategies.py b/swh/model/hypothesis_strategies.py
--- a/swh/model/hypothesis_strategies.py
+++ b/swh/model/hypothesis_strategies.py
@@ -34,6 +34,7 @@
     Content,
     Directory,
     DirectoryEntry,
+    ExtID,
     ObjectType,
     Origin,
     OriginVisit,
@@ -287,6 +288,20 @@
     return one_of(present_contents(), skipped_contents())
 
 
+def extids_d():
+    return builds(
+        dict,
+        target_type=sampled_from([x.value for x in ObjectType]),
+        target=sha1_git(),
+        extid_type=sampled_from(["hg", "git", "bzr"]),
+        extid=sha1(),
+    )
+
+
+def extids():
+    return extids_d().map(ExtID.from_dict)
+
+
 def present_contents_d():
     return builds(
         dict,
@@ -421,6 +436,7 @@
         ("release", releases),
         ("revision", revisions),
         ("directory", directories),
+        ("extid", extids),
     ]
     if split_content:
         strategies.append(("content", present_contents))
@@ -454,6 +470,7 @@
         ("release", releases_d),
         ("revision", revisions_d),
         ("directory", directories_d),
+        ("extid", extids_d),
     ]
     if split_content:
         strategies.append(("content", present_contents_d))
diff --git a/swh/model/model.py b/swh/model/model.py
--- a/swh/model/model.py
+++ b/swh/model/model.py
@@ -35,7 +35,7 @@
     pass
 
 
-KeyType = Union[Dict[str, str], Dict[str, bytes], bytes]
+KeyType = Union[Dict[str, str], Dict[str, bytes], Tuple[Union[str, bytes], ...], bytes]
 """The type returned by BaseModel.unique_key()."""
 
 
@@ -1072,3 +1072,25 @@
             "fetcher_name": self.fetcher.name,
             "fetcher_version": self.fetcher.version,
         }
+
+
+@attr.s(frozen=True, slots=True)
+class ExtID(BaseModel):
+    object_type: Final = "extid"
+
+    extid = attr.ib(type=bytes, validator=type_validator())
+    extid_type = attr.ib(type=str, validator=type_validator())
+    target = attr.ib(type=Sha1Git, validator=type_validator())
+    target_type = attr.ib(type=ObjectType, validator=type_validator())
+
+    @classmethod
+    def from_dict(cls, d):
+        return cls(
+            extid=d["extid"],
+            extid_type=d["extid_type"],
+            target=d["target"],
+            target_type=ObjectType(d["target_type"]),
+        )
+
+    def unique_key(self) -> KeyType:
+        return attr.astuple(self)
diff --git a/swh/model/tests/test_model.py b/swh/model/tests/test_model.py
--- a/swh/model/tests/test_model.py
+++ b/swh/model/tests/test_model.py
@@ -1224,3 +1224,16 @@
             ),
             **_common_metadata_fields,
         )
+
+
+@given(strategies.extids())
+def test_extid(extid):
+    assert extid
+    assert extid.unique_key()
+    assert extid.target_type.value in (
+        "content",
+        "directory",
+        "revision",
+        "release",
+        "snapshot",
+    )