diff --git a/frontend/src/pages/entity-configuration.vue b/frontend/src/pages/entity-configuration.vue
index 6a0537a..c46a42c 100644
--- a/frontend/src/pages/entity-configuration.vue
+++ b/frontend/src/pages/entity-configuration.vue
@@ -149,6 +149,14 @@
     editingFieldIdentifier.value = toIdentifier(editingFieldName.value)
   }
 
+  function updateNewFieldType (entityId: number) {
+    newFields[entityId].relationshipType = newFields[entityId].type === 'USER' ? 'ONE_TO_MANY' : 'ONE_TO_ONE'
+  }
+
+  function updateEditingFieldType () {
+    editingRelationshipType.value = editingFieldType.value === 'USER' ? 'ONE_TO_MANY' : 'ONE_TO_ONE'
+  }
+
   async function addEntity () {
     const name = newEntityName.value.trim()
     const identifier = newEntityIdentifier.value.trim()
@@ -208,6 +216,8 @@
         if (!draft.targetEntityId) return
         values.targetEntityId = String(draft.targetEntityId)
         values.relationshipType = draft.relationshipType
+      } else if (draft.type === 'USER') {
+        values.relationshipType = draft.relationshipType
       }
       const response = await request(`/api/entity-definitions/${entity.id}/fields`, 'POST', values)
       if (!response.ok) throw new Error('Field create failed')
@@ -227,7 +237,7 @@
     editingFieldIdentifier.value = field.identifier
     editingFieldType.value = field.type
     editingTargetEntityId.value = field.targetEntityId
-    editingRelationshipType.value = field.relationshipType ?? 'ONE_TO_ONE'
+    editingRelationshipType.value = field.relationshipType ?? (field.type === 'USER' ? 'ONE_TO_MANY' : 'ONE_TO_ONE')
   }
 
   async function saveField (entity: EntityDefinition, field: EntityField) {
@@ -244,6 +254,8 @@
         if (!editingTargetEntityId.value) return
         values.targetEntityId = String(editingTargetEntityId.value)
         values.relationshipType = editingRelationshipType.value
+      } else if (editingFieldType.value === 'USER') {
+        values.relationshipType = editingRelationshipType.value
       }
       const response = await request(`/api/entity-definitions/${entity.id}/fields/${field.id}`, 'PATCH', values)
       if (!response.ok) throw new Error('Field update failed')
@@ -346,6 +358,10 @@
     return type === 'ONE_TO_MANY' ? 'One to many' : 'One to one'
   }
 
+  function userRelationshipLabel (type?: RelationshipType) {
+    return type === 'ONE_TO_ONE' ? 'One to one' : 'Many to one'
+  }
+
   function targetName (field: EntityField) {
     return field.targetEntityName
       ?? entities.value.find(entity => entity.id === field.targetEntityId)?.name
@@ -488,7 +504,7 @@
             <input :id="`edit-field-identifier-${field.id}`" v-model="editingFieldIdentifier" required>
             <label class="sr-only" :for="`edit-type-${field.id}`">Field type</label>
 
-            <select :id="`edit-type-${field.id}`" v-model="editingFieldType">
+            <select :id="`edit-type-${field.id}`" v-model="editingFieldType" @change="updateEditingFieldType">
               <option v-for="type in fieldTypes" :key="type" :value="type">{{ typeLabel(type) }}</option>
             </select>
 
@@ -508,6 +524,15 @@
               </select>
             </template>
 
+            <template v-else-if="editingFieldType === 'USER'">
+              <label class="sr-only" :for="`edit-user-relationship-${field.id}`">User relationship type</label>
+
+              <select :id="`edit-user-relationship-${field.id}`" v-model="editingRelationshipType">
+                <option value="ONE_TO_MANY">Many to one</option>
+                <option value="ONE_TO_ONE">One to one</option>
+              </select>
+            </template>
+
             <button class="text-button" type="submit">Save field</button>
             <button class="text-button muted" type="button" @click="editingFieldId = undefined">Cancel</button>
           </form>
@@ -522,7 +547,9 @@
                 {{ targetName(field) }} · {{ relationshipLabel(field.relationshipType) }}
               </span>
 
-              <span v-else-if="field.type === 'USER'" class="reference-detail">Built-in User entity</span>
+              <span v-else-if="field.type === 'USER'" class="reference-detail">
+                Built-in User entity · {{ userRelationshipLabel(field.relationshipType) }}
+              </span>
             </div>
 
             <div class="actions">
@@ -549,7 +576,7 @@
           <input :id="`new-field-identifier-${entity.id}`" v-model="newFields[entity.id].identifier" placeholder="Identifier" required>
           <label class="sr-only" :for="`new-type-${entity.id}`">Field type</label>
 
-          <select :id="`new-type-${entity.id}`" v-model="newFields[entity.id].type">
+          <select :id="`new-type-${entity.id}`" v-model="newFields[entity.id].type" @change="updateNewFieldType(entity.id)">
             <option v-for="type in fieldTypes" :key="type" :value="type">{{ typeLabel(type) }}</option>
           </select>
 
@@ -569,6 +596,15 @@
             </select>
           </template>
 
+          <template v-else-if="newFields[entity.id].type === 'USER'">
+            <label class="sr-only" :for="`new-user-relationship-${entity.id}`">User relationship type</label>
+
+            <select :id="`new-user-relationship-${entity.id}`" v-model="newFields[entity.id].relationshipType">
+              <option value="ONE_TO_MANY">Many to one</option>
+              <option value="ONE_TO_ONE">One to one</option>
+            </select>
+          </template>
+
           <button class="secondary-button" :disabled="saving" type="submit">Add field</button>
         </div>
       </form>
diff --git a/frontend/tests/entity-configuration.spec.ts b/frontend/tests/entity-configuration.spec.ts
index 2f87917..9a443d1 100644
--- a/frontend/tests/entity-configuration.spec.ts
+++ b/frontend/tests/entity-configuration.spec.ts
@@ -184,9 +184,10 @@ test('admin can add, edit, and remove an entity and its fields', async ({ page }
 
   await organizationFieldForm.getByLabel('New field').fill('Account owner')
   await organizationFieldForm.getByLabel('Field type').selectOption('USER')
+  await organizationFieldForm.getByLabel('User relationship type').selectOption('ONE_TO_ONE')
   await organizationFieldForm.getByRole('button', { name: 'Add field' }).click()
   await expect(organization.locator('.type-badge').filter({ hasText: 'User reference' })).toBeVisible()
-  await expect(organization.getByText('Built-in User entity')).toBeVisible()
+  await expect(organization.getByText('Built-in User entity · One to one')).toBeVisible()
 
   await organization.getByRole('button', { name: 'Remove entity' }).click()
   await expect(page.getByRole('heading', { name: 'Organization' })).not.toBeVisible()
diff --git a/src/main/kotlin/dev/mduchene/bolts/web/EntityDefinitionController.kt b/src/main/kotlin/dev/mduchene/bolts/web/EntityDefinitionController.kt
index ec7d56a..d113a33 100644
--- a/src/main/kotlin/dev/mduchene/bolts/web/EntityDefinitionController.kt
+++ b/src/main/kotlin/dev/mduchene/bolts/web/EntityDefinitionController.kt
@@ -140,6 +140,11 @@ class EntityDefinitionController(
     }
 
     private fun Context.relationshipDetails(fieldType: FieldType?): RelationshipDetails? {
+        if (fieldType == FieldType.USER) {
+            val relationshipType = formParam("relationshipType")?.let(RelationshipType::from)
+                ?: RelationshipType.ONE_TO_MANY
+            return RelationshipDetails(null, relationshipType)
+        }
         if (fieldType != FieldType.RELATIONSHIP) return RelationshipDetails(null, null)
 
         val targetEntityId = formParam("targetEntityId")?.toLongOrNull()