diff --git a/latest_migrations.manifest b/latest_migrations.manifest
index ed1766c1754ab..403e9a98b3f04 100644
--- a/latest_migrations.manifest
+++ b/latest_migrations.manifest
@@ -5,7 +5,7 @@ contenttypes: 0002_remove_content_type_name
 ee: 0015_add_verified_properties
 otp_static: 0002_throttling
 otp_totp: 0002_auto_20190420_0723
-posthog: 0396_projects_and_environments
+posthog: 0397_projects_backfill
 sessions: 0001_initial
 social_django: 0010_uid_db_index
 two_factor: 0007_auto_20201201_1019
diff --git a/plugin-server/functional_tests/api.ts b/plugin-server/functional_tests/api.ts
index 539fa266b8dd4..abbd770d7bb77 100644
--- a/plugin-server/functional_tests/api.ts
+++ b/plugin-server/functional_tests/api.ts
@@ -365,8 +365,18 @@ export const createTeam = async (
     token?: string,
     sessionRecordingOptIn = true
 ) => {
-    const team = await insertRow(postgres, 'posthog_team', {
+    const id = Math.round(Math.random() * 1000000000)
+    await insertRow(postgres, 'posthog_project', {
+        // Every team (aka environment) must be a child of a project
+        id,
         organization_id: organizationId,
+        name: 'TEST PROJECT',
+        created_at: new Date().toISOString(),
+    })
+    await insertRow(postgres, 'posthog_team', {
+        id,
+        organization_id: organizationId,
+        project_id: id,
         app_urls: [],
         name: 'TEST PROJECT',
         event_names: [],
@@ -392,7 +402,7 @@ export const createTeam = async (
         access_control: false,
         slack_incoming_webhook,
     })
-    return team.id
+    return id
 }
 
 export const createAction = async (action: Omit<RawAction, 'id'>, steps: Omit<ActionStep, 'id' | 'action_id'>[]) => {
diff --git a/plugin-server/tests/helpers/sql.ts b/plugin-server/tests/helpers/sql.ts
index eb167cccdb20f..8564250118550 100644
--- a/plugin-server/tests/helpers/sql.ts
+++ b/plugin-server/tests/helpers/sql.ts
@@ -224,8 +224,15 @@ export async function createUserTeamAndOrganization(
         joined_at: new Date().toISOString(),
         updated_at: new Date().toISOString(),
     })
+    await insertRow(db, 'posthog_project', {
+        id: teamId,
+        organization_id: organizationId,
+        name: 'TEST PROJECT',
+        created_at: new Date().toISOString(),
+    })
     await insertRow(db, 'posthog_team', {
         id: teamId,
+        project_id: teamId,
         organization_id: organizationId,
         app_urls: [],
         name: 'TEST PROJECT',
@@ -315,10 +322,19 @@ export const createOrganization = async (pg: PostgresRouter) => {
 }
 
 export const createTeam = async (pg: PostgresRouter, organizationId: string, token?: string) => {
-    const team = await insertRow(pg, 'posthog_team', {
-        // KLUDGE: auto increment IDs can be racy in tests so we ensure IDs don't clash
-        id: Math.round(Math.random() * 1000000000),
+    // KLUDGE: auto increment IDs can be racy in tests so we ensure IDs don't clash
+    const id = Math.round(Math.random() * 1000000000)
+    await insertRow(pg, 'posthog_project', {
+        // Every team (aka environment) must be a child of a project
+        id,
+        organization_id: organizationId,
+        name: 'TEST PROJECT',
+        created_at: new Date().toISOString(),
+    })
+    await insertRow(pg, 'posthog_team', {
+        id,
         organization_id: organizationId,
+        project_id: id,
         app_urls: [],
         name: 'TEST PROJECT',
         event_names: [],
@@ -343,7 +359,7 @@ export const createTeam = async (pg: PostgresRouter, organizationId: string, tok
         person_display_name_properties: [],
         access_control: false,
     })
-    return team.id
+    return id
 }
 
 export const createUser = async (pg: PostgresRouter, distinctId: string) => {
diff --git a/posthog/migrations/0397_projects_backfill.py b/posthog/migrations/0397_projects_backfill.py
new file mode 100644
index 0000000000000..5b3da405af116
--- /dev/null
+++ b/posthog/migrations/0397_projects_backfill.py
@@ -0,0 +1,41 @@
+# Generated by Django 4.1.13 on 2024-03-12 23:14
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("posthog", "0396_projects_and_environments"),
+    ]
+
+    operations = [
+        migrations.SeparateDatabaseAndState(
+            database_operations=[
+                migrations.RunSQL(
+                    sql="""
+                    -- For each team without a parent project, create such a project
+                    INSERT INTO posthog_project (id, name, created_at, organization_id)
+                    SELECT id, name, created_at, organization_id
+                    FROM posthog_team
+                    WHERE project_id IS NULL;
+                    -- At this point, all teams have a parent project, so we can safely set project_id on every team
+                    UPDATE posthog_team
+                    SET project_id = id;""",
+                    reverse_sql=migrations.RunSQL.noop,
+                )
+            ],
+            state_operations=[
+                migrations.AlterField(
+                    model_name="team",
+                    name="project",
+                    field=models.ForeignKey(
+                        on_delete=django.db.models.deletion.CASCADE,
+                        related_name="teams",
+                        related_query_name="team",
+                        to="posthog.project",
+                    ),
+                ),
+            ],
+        )
+    ]
diff --git a/posthog/models/team/team.py b/posthog/models/team/team.py
index 93a486cdc98fc..66b4a3ed51415 100644
--- a/posthog/models/team/team.py
+++ b/posthog/models/team/team.py
@@ -175,8 +175,6 @@ class Team(UUIDClassicModel):
         on_delete=models.CASCADE,
         related_name="teams",
         related_query_name="team",
-        null=True,
-        blank=False,
     )
     api_token: models.CharField = models.CharField(
         max_length=200,