diff --git a/CONTRIBUTORS b/CONTRIBUTORS
--- a/CONTRIBUTORS
+++ b/CONTRIBUTORS
@@ -3,4 +3,5 @@
 Léni Gauffier
 Yann Gautier
 Sushant Sushant
-Hezekiah Maina
\ No newline at end of file
+Hezekiah Maina
+Léo Andrès
diff --git a/setup.py b/setup.py
--- a/setup.py
+++ b/setup.py
@@ -65,6 +65,7 @@
         lister.gnu=swh.lister.gnu:register
         lister.launchpad=swh.lister.launchpad:register
         lister.npm=swh.lister.npm:register
+        lister.opam=swh.lister.opam:register
         lister.packagist=swh.lister.packagist:register
         lister.phabricator=swh.lister.phabricator:register
         lister.pypi=swh.lister.pypi:register
diff --git a/swh/lister/opam/__init__.py b/swh/lister/opam/__init__.py
new file mode 100644
--- /dev/null
+++ b/swh/lister/opam/__init__.py
@@ -0,0 +1,12 @@
+# Copyright (C) 2019-2021 the Software Heritage developers
+# License: GNU General Public License version 3, or any later version
+# See top-level LICENSE file for more information
+
+
+def register():
+    from .lister import OpamLister
+
+    return {
+        "lister": OpamLister,
+        "task_modules": ["%s.tasks" % __name__],
+    }
diff --git a/swh/lister/opam/lister.py b/swh/lister/opam/lister.py
new file mode 100644
--- /dev/null
+++ b/swh/lister/opam/lister.py
@@ -0,0 +1,82 @@
+# Copyright (C) 2021  The Software Heritage developers
+# See the AUTHORS file at the top-level directory of this distribution
+# License: GNU General Public License version 3, or any later version
+# See top-level LICENSE file for more information
+
+import io
+import logging
+import os
+from subprocess import PIPE, Popen, call
+import tempfile
+from typing import Iterator
+
+from swh.lister.pattern import StatelessLister
+from swh.scheduler.interface import SchedulerInterface
+from swh.scheduler.model import ListedOrigin
+
+from ..pattern import CredentialsType
+
+logger = logging.getLogger(__name__)
+
+PageType = str
+
+
+class OpamLister(StatelessLister[PageType]):
+    """
+    List all repositories hosted on an opam repository.
+
+    Args:
+        url: base URL of an opam repository
+            (for instance https://opam.ocaml.org)
+        instance: string identifier for the listed repository
+
+    """
+
+    # Part of the lister API, that identifies this lister
+    LISTER_NAME = "opam"
+
+    def __init__(
+        self,
+        scheduler: SchedulerInterface,
+        url: str,
+        instance: str,
+        credentials: CredentialsType = None,
+    ):
+        super().__init__(
+            scheduler=scheduler, credentials=credentials, url=url, instance=instance,
+        )
+        self.env = os.environ.copy()
+        self.env["OPAMROOT"] = tempfile.mkdtemp(prefix="swh_opam_lister")
+        call(
+            ["opam", "init", "--reinit", "--bare", "--no-setup", instance, url],
+            env=self.env,
+        )
+
+    def get_pages(self) -> Iterator[PageType]:
+        proc = Popen(
+            [
+                "opam",
+                "list",
+                "--all",
+                "--no-switch",
+                "--repos",
+                self.instance,
+                "--normalise",
+                "--short",
+            ],
+            env=self.env,
+            stdout=PIPE,
+        )
+        if proc.stdout is not None:
+            for line in io.TextIOWrapper(proc.stdout):
+                yield line.rstrip("\n")
+
+    def get_origins_from_page(self, page: PageType) -> Iterator[ListedOrigin]:
+        """Convert a page of OpamLister repositories into a list of ListedOrigins"""
+        assert self.lister_obj.id is not None
+        # a page is just a package name
+        url = f"opam+{self.url}/packages/{page}/"
+        # print("adding url", url)
+        yield ListedOrigin(
+            lister_id=self.lister_obj.id, visit_type="opam", url=url, last_update=None
+        )
diff --git a/swh/lister/opam/tasks.py b/swh/lister/opam/tasks.py
new file mode 100644
--- /dev/null
+++ b/swh/lister/opam/tasks.py
@@ -0,0 +1,18 @@
+# Copyright (C) 2019 the Software Heritage developers
+# License: GNU General Public License version 3, or any later version
+# See top-level LICENSE file for more information
+
+from celery import shared_task
+
+from swh.lister.opam.lister import OpamLister
+
+
+@shared_task(name=__name__ + ".OpamListerTask")
+def list_opam(**lister_args):
+    """Lister task for the Opam registry"""
+    return OpamLister.from_configfile(**lister_args).run().dict()
+
+
+@shared_task(name=__name__ + ".ping")
+def _ping():
+    return "OK"
diff --git a/swh/lister/tests/test_cli.py b/swh/lister/tests/test_cli.py
--- a/swh/lister/tests/test_cli.py
+++ b/swh/lister/tests/test_cli.py
@@ -16,6 +16,7 @@
     },
     "gitea": {"url": "https://try.gitea.io/api/v1/",},
     "gitlab": {"url": "https://gitlab.ow2.org/api/v4", "instance": "ow2",},
+    "opam": {"url": "https://opam.ocaml.org", "instance": "opam"},
 }