diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0ba047b..621fb4f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,7 @@ and this project adheres to
 [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## Unreleased
+- map SAML entity IDs to SIRET (#34)
 - include ACR claim in ID token only if requested (#32)
 
 ## [1.0.8] - 2025-09-10
diff --git a/docker-compose.yml b/docker-compose.yml
index ec1f90f..0a447ef 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -66,7 +66,7 @@ services:
       #OIDC_PROVIDER: https://oidc2fer-staging.beta.numerique.gouv.fr
       OIDC_CLIENT_ID: oidc-test-client
       OIDC_CLIENT_SECRET: oidc-test-secret
-      OIDC_SCOPES: openid,uid,given_name,usual_name,email
+      OIDC_SCOPES: openid,uid,given_name,usual_name,email,siret
     extra_hosts:
       - "oidc2fer.127.0.0.1.nip.io:host-gateway"
 
diff --git a/src/helm/env.d/dev/values.oidc2fer.yaml.gotmpl b/src/helm/env.d/dev/values.oidc2fer.yaml.gotmpl
index 7301ebe..2d38b2c 100644
--- a/src/helm/env.d/dev/values.oidc2fer.yaml.gotmpl
+++ b/src/helm/env.d/dev/values.oidc2fer.yaml.gotmpl
@@ -36,6 +36,10 @@ satosa:
           "token_endpoint_auth_method": "client_secret_post"
         }
       }
+    SIRET_MAP: |-
+      {
+          "https://test-idp.federation.renater.fr/idp/shibboleth": "12345678200010"
+      }
 
 ingress:
   enabled: true
diff --git a/src/helm/env.d/outscale-production/values.oidc2fer.yaml.gotmpl b/src/helm/env.d/outscale-production/values.oidc2fer.yaml.gotmpl
index e006f66..6659212 100644
--- a/src/helm/env.d/outscale-production/values.oidc2fer.yaml.gotmpl
+++ b/src/helm/env.d/outscale-production/values.oidc2fer.yaml.gotmpl
@@ -24,6 +24,94 @@ satosa:
 
     OIDC_DB_URI: { secretKeyRef: { name: redis.redis.libre.sh, key: url } }
 
+    # As provided by RENATER on 2025-09-19
+    SIRET_MAP: |-
+      {
+        "https://shibboleth.grenoble-inp.fr/idp/shibboleth": "19381912500017",
+        "https://identites.ensea.fr/idp/shibboleth": "19951376300011",
+        "https://idp.ent.dauphine.fr/idp/shibboleth": "19754692200018",
+        "https://idp.univ-lyon2.fr/idp/shibboleth": "19691775100014",
+        "https://shibboleth.insa-rouen.fr/idp/shibboleth": "19760165100023",
+        "https://idp1.univ-fcomte.fr/idp/shibboleth": "19251215000363",
+        "https://idp.univ-tours.fr/idp/shibboleth": "19370800500478",
+        "https://apps.univ-lr.fr/idp/shibboleth": "19170032700015",
+        "https://shib.mines-albi.fr/idp/shibboleth": "19811200500022",
+        "https://federation.upf.pf/idp/shibboleth": "19987001500013",
+        "https://federation.utbm.fr/idp/shibboleth": "19900356700013",
+        "urn:mace:cru.fr:federation:univ-paris1.fr": "19751717000019",
+        "https://idp.sciencespobordeaux.fr/idp/shibboleth": "19330192600039",
+        "https://vip.espci.fr/saml2/idp/metadata.php": "20000068500012",
+        "urn:mace:cru.fr:federation:univ-nantes.fr": "13002974700016",
+        "https://shibboleth.univ-corse.fr/idp/shibboleth": "19202664900264",
+        "https://srv-fii.insa-toulouse.fr/idp/shibboleth": "19310152400018",
+        "urn:mace:cru.fr:federation:univ-rouen.fr": "19761904200017",
+        "https://ident-shib.ensc-rennes.fr/idp/shibboleth": "19350077400016",
+        "urn:mace:cru.fr:federation:unilim.fr": "19870669900321",
+        "https://idp.inha.fr/idp/shibboleth": "19754688000018",
+        "https://federation.unimes.fr/idp/shibboleth": "13000375900029",
+        "urn:mace:cru.fr:federation:univ-rennes1.fr": "13003051300019",
+        "urn:mace:cru.fr:federation:univ-ubs.fr": "19561718800600",
+        "https://idp.univ-orleans.fr/idp/shibboleth": "19450855200016",
+        "https://shibboleth.univ-savoie.fr/idp/shibboleth": "19730858800015",
+        "https://idp.cirad.fr/idp/shibboleth": "33159627000016",
+        "https://sso.ird.fr/idp/shibboleth": "18000602500159",
+        "https://idp.ensma.fr/idp/shibboleth": "19860073600021",
+        "https://idp.renater.fr/idp/shibboleth": "18008947600055",
+        "https://idp.inp-toulouse.fr/idp/shibboleth": "19311381800127",
+        "https://idp.unistra.fr/idp/shibboleth": "13000545700010",
+        "https://idp1.agroparistech.fr/idp/shibboleth": "13000285000134",
+        "https://janus.cnrs.fr/idp": "18008901303720",
+        "https://federation.umontpellier.fr/idp/shibboleth": "13002979600013",
+        "https://idp.sciencespo-lyon.fr/idp/shibboleth": "19690173000024",
+        "https://shibboleth.univ-grenoble-alpes.fr/idp/shibboleth": "13002608100013",
+        "https://idp.bnu.fr/idp/shibboleth": "18004406700015",
+        "https://ruhnu.univ-tlse2.fr/idp/shibboleth": "19311383400017",
+        "https://idp2.amue.fr/idp/shibboleth": "18004312700083",
+        "https://idpv3.univ-amu.fr/idp/shibboleth": "13001533200013",
+        "https://idp-ng.univ-st-etienne.fr/idp/shibboleth": "93850168100010",
+        "https://identities.univ-jfc.fr/idp/prod": "19811201300018",
+        "https://idp3.univ-lorraine.fr/idp/shibboleth": "13001550600012",
+        "https://idp.univ-lemans.fr/idp/shibboleth": "19720916600010",
+        "https://idp.univ-lille.fr/idp/shibboleth": "13002975400012",
+        "https://idp.univ-tln.fr/idp/shibboleth": "19830766200017",
+        "https://idp.uca.fr/idp/shibboleth": "13002806100013",
+        "https://shibboleth3.utt.fr/idp/shibboleth": "19101060200032",
+        "https://idp3.ut-capitole.fr/idp/shibboleth": "19311382600013",
+        "https://idp.imt-atlantique.fr/idp/shibboleth": "18009202500121",
+        "https://shib.univ-lehavre.fr/idp/shibboleth": "19762762300097",
+        "https://idp2.univ-paris8.fr/idp/shibboleth": "19931827000014",
+        "https://shib2.unc.nc/idp/shibboleth": "13000322100012",
+        "https://upnidp2.parisnanterre.fr/idp/shibboleth": "19921204400010",
+        "https://idp.ensfea.fr/idp/shibboleth": "19310143300012",
+        "https://idp.univ-eiffel.fr/idp/shibboleth": "13002612300013",
+        "https://idp.insa-rennes.fr/idp/shibboleth": "19350097200016",
+        "https://idp.univ-tlse3.fr/idp/shibboleth": "93827139200012",
+        "https://idp.universite-paris-saclay.fr/idp": "13002602400054",
+        "https://shibboleth.ens2m.fr/idp/shibboleth": "19250082500026",
+        "https://sso-ciation-saclay.centralesupelec.fr/idp/shibboleth": "13002076100016",
+        "https://federation.ens.psl.eu/idp/shibboleth": "19753459700012",
+        "https://authentification.inrae.fr/saml/metadata/idp": "18007003901803",
+        "https://idp.ensam.eu/idp/shibboleth": "19753472000010",
+        "https://idp1.ens-paris-saclay.fr/idp/shibboleth": "19940607500036",
+        "https://idp2.emse.fr/idp/shibboleth": "18009202500105",
+        "https://idp-septa.onisep.fr/idp/shibboleth": "18004302800653",
+        "https://papangue.vetagro-sup.fr/idp/shibboleth": "13000858400018",
+        "https://multipass.imt-nord-europe.fr/idp/shibboleth": "18009202500139",
+        "https://idp4.unicaen.fr/idp/shibboleth": "19141408500016",
+        "https://sso.bordeaux-inp.fr/idp/shibboleth": "13000635600013",
+        "https://auth.uttop.fr/saml/metadata": "19650048200019",
+        "https://idp.univ-cotedazur.fr/idp/shibboleth": "13002566100013",
+        "https://idp2.brgm.fr/idp/shibboleth": "58205614900120",
+        "https://idp.crous-reims.fr/idp/shibboleth": "18510200100012",
+        "https://idp.univ-avignon.fr/idp/shibboleth": "19840685200204",
+        "https://idp-genci.renater.fr/idp/shibboleth": "49468697500041",
+        "https://auth.sso.vet-alfort.fr/saml/metadata": "19940608300014",
+        "https://idp.univ-perp.fr/idp/shibboleth": "19660437500010",
+        "https://idp.institut-agro.fr/idp": "13000279300011",
+        "https://prod-idp.inria.fr": "18008904700013",
+        "https://idp.ec-nantes.fr/idp/shibboleth": "19440100600011"
+      }
+
 ingress:
   enabled: true
   host: renater.agentconnect.gouv.fr
diff --git a/src/helm/env.d/staging/values.oidc2fer.yaml.gotmpl b/src/helm/env.d/staging/values.oidc2fer.yaml.gotmpl
index c8964af..c5821eb 100644
--- a/src/helm/env.d/staging/values.oidc2fer.yaml.gotmpl
+++ b/src/helm/env.d/staging/values.oidc2fer.yaml.gotmpl
@@ -24,6 +24,11 @@ satosa:
 
     OIDC_DB_URI: { secretKeyRef: { name: redis.redis.libre.sh, key: url } }
 
+    SIRET_MAP: |-
+      {
+          "https://test-idp.federation.renater.fr/idp/shibboleth": "12345678200010"
+      }
+
 ingress:
   enabled: true
   host: oidc2fer-staging.beta.numerique.gouv.fr
diff --git a/src/satosa/internal_attributes.yaml b/src/satosa/internal_attributes.yaml
index 6a76b29..a3b6e7a 100644
--- a/src/satosa/internal_attributes.yaml
+++ b/src/satosa/internal_attributes.yaml
@@ -31,3 +31,6 @@ attributes:
   uid:
     openid:
       - uid
+  siret:
+    openid:
+      - siret
diff --git a/src/satosa/oidc2fer/attribute_generators/__init__.py b/src/satosa/oidc2fer/attribute_generators/__init__.py
new file mode 100644
index 0000000..73c690f
--- /dev/null
+++ b/src/satosa/oidc2fer/attribute_generators/__init__.py
@@ -0,0 +1 @@
+from .entity_id_to_siret_mapper import EntityIdToSiretMapper
diff --git a/src/satosa/oidc2fer/attribute_generators/entity_id_to_siret_mapper.py b/src/satosa/oidc2fer/attribute_generators/entity_id_to_siret_mapper.py
new file mode 100644
index 0000000..0b05eac
--- /dev/null
+++ b/src/satosa/oidc2fer/attribute_generators/entity_id_to_siret_mapper.py
@@ -0,0 +1,23 @@
+import json
+import logging
+
+from satosa.micro_services.base import ResponseMicroService
+
+logger = logging.getLogger(__name__)
+
+
+class EntityIdToSiretMapper(ResponseMicroService):
+    def __init__(self, config, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        self.attribute = config.get("attribute", "siret")
+        self.mapping = json.loads(config.get("mapping_json", "{}"))
+
+    def process(self, context, data):
+        entity_id = data.auth_info.issuer
+        if entity_id in self.mapping:
+            siret = self.mapping[entity_id]
+            logger.info("Mapping entity ID %s to SIRET %s", entity_id, siret)
+            data.attributes[self.attribute] = siret
+        else:
+            logger.warning("No SIRET mapping found for entity ID %s", entity_id)
+        return super().process(context, data)
diff --git a/src/satosa/plugins/frontends/openid_connect_frontend.yaml b/src/satosa/plugins/frontends/openid_connect_frontend.yaml
index 9505bad..f14c718 100644
--- a/src/satosa/plugins/frontends/openid_connect_frontend.yaml
+++ b/src/satosa/plugins/frontends/openid_connect_frontend.yaml
@@ -43,6 +43,7 @@ config:
       - uid
       - given_name
       - usual_name
+      - siret
 
     # Set code/token lifetimes as short as possible
     authorization_code_lifetime: 60
@@ -56,3 +57,5 @@ config:
       - given_name
       usual_name:
       - usual_name
+      siret:
+      - siret
diff --git a/src/satosa/plugins/microservices/siret_mapping.yaml b/src/satosa/plugins/microservices/siret_mapping.yaml
new file mode 100644
index 0000000..ef00bfc
--- /dev/null
+++ b/src/satosa/plugins/microservices/siret_mapping.yaml
@@ -0,0 +1,5 @@
+module: oidc2fer.attribute_generators.EntityIdToSiretMapper
+name: EntityIdToSiretMapper
+config:
+  attribute: siret
+  mapping_json: !ENV SIRET_MAP
diff --git a/src/satosa/proxy_conf.yaml b/src/satosa/proxy_conf.yaml
index c6bcca1..c8127bd 100644
--- a/src/satosa/proxy_conf.yaml
+++ b/src/satosa/proxy_conf.yaml
@@ -17,6 +17,7 @@ MICRO_SERVICES:
   - plugins/microservices/primary_identifier.yaml
   - plugins/microservices/static_attributes.yaml
   - plugins/microservices/attribute_processor.yaml
+  - plugins/microservices/siret_mapping.yaml
 LOGGING:
   # All the logging configuration is done in the Gunicorn config, this is just
   # here to avoid overwriting it with the default SATOSA config
diff --git a/src/satosa/tests/oidc2fer/attribute_generators/test_entity_id_to_siret_mapper.py b/src/satosa/tests/oidc2fer/attribute_generators/test_entity_id_to_siret_mapper.py
new file mode 100644
index 0000000..2065f86
--- /dev/null
+++ b/src/satosa/tests/oidc2fer/attribute_generators/test_entity_id_to_siret_mapper.py
@@ -0,0 +1,42 @@
+import json
+
+from satosa.context import Context
+from satosa.internal import AuthenticationInformation, InternalData
+
+from oidc2fer.attribute_generators import EntityIdToSiretMapper
+
+
+class TestEntityIdToSiretMapper:
+    def create_mapper(self):
+        mapper = EntityIdToSiretMapper(
+            config={
+                "attribute": "siret",
+                "mapping_json": json.dumps(
+                    {
+                        "https://idp.example.fr": "12345678200010",
+                    }
+                ),
+            },
+            name="siret_mapper",
+            base_url="https://satosa.example.com",
+        )
+        mapper.next = lambda ctx, data: data
+        return mapper
+
+    def test_sets_siret_for_known_entityid(self):
+        mapper = self.create_mapper()
+        resp = InternalData(
+            auth_info=AuthenticationInformation(issuer="https://idp.example.fr")
+        )
+        ctx = Context()
+        mapper.process(ctx, resp)
+        assert resp.attributes["siret"] == "12345678200010"
+
+    def test_does_not_set_siret_for_unknown_entityid(self):
+        mapper = self.create_mapper()
+        resp = InternalData(
+            auth_info=AuthenticationInformation(issuer="https://unknown.example.fr")
+        )
+        ctx = Context()
+        mapper.process(ctx, resp)
+        assert "siret" not in resp.attributes
diff --git a/src/satosa/tests/test_e2e.py b/src/satosa/tests/test_e2e.py
index cef255d..b208eff 100644
--- a/src/satosa/tests/test_e2e.py
+++ b/src/satosa/tests/test_e2e.py
@@ -49,6 +49,7 @@ def oidc_to_renater(
         "email": expected_email,
         "given_name": expected_given_name,
         "usual_name": expected_usual_name,
+        "siret": "12345678200010",
     }.items() <= userinfo.items()
     return id_token
 
@@ -102,6 +103,7 @@ def pro_connect_to_renater(
         "email": expected_email,
         "given_name": expected_given_name,
         "usual_name": expected_usual_name,
+        "siret": "12345678200010",
     }.items() <= result.items()
     return result