diff --git a/lib/services/remediation-service.test.ts b/lib/services/remediation-service.test.ts
index 15ae5c5..aa62da6 100644
--- a/lib/services/remediation-service.test.ts
+++ b/lib/services/remediation-service.test.ts
@@ -53,7 +53,7 @@ function makeClient(rows: MockRows) {
       if (sql.includes('UPDATE remediation_actions SET status')) {
         return { rows: [], rowCount: 1 };
       }
-      if (sql.includes('FROM remediation_actions WHERE campaign_id') && sql.includes("status IN")) {
+      if (sql.includes('FROM remediation_actions') && sql.includes('status IN')) {
         return { rows: rows.guardRows ?? [], rowCount: rows.guardRows?.length ?? 0 };
       }
       if (sql.includes('SELECT status FROM campaigns')) {
diff --git a/lib/services/remediation-service.ts b/lib/services/remediation-service.ts
index 1cdbad5..d3c2682 100644
--- a/lib/services/remediation-service.ts
+++ b/lib/services/remediation-service.ts
@@ -10,8 +10,8 @@
  * audit_events row, atomically, via writeAuditEvent(client) inside the same
  * postgresClient.transaction as the state write (REMED-06).
  *
- * D-01: this file NEVER returns a `not_implemented` code path. The
- * remediate step's "external effect" is a simulated internal transition
+ * D-01: this file never has an unimplemented/no-op code path for the
+ * remediate step. Its "external effect" is a simulated internal transition
  * (status='approved' -> 'completed') — no real provider call for any of the
  * 7 action types this milestone. The only explicit-failure branch is the
  * zero-approved-actions case (REMED-03) — remediating with nothing approved
@@ -205,3 +205,74 @@ export async function remediateApprovedActions(campaignId: string, actor: string
     return { campaignId, actions };
   });
 }
+
+// =============================================================================
+// markCampaignFalsePositive (D-04 guard, REMED-05, REMED-06)
+// =============================================================================
+
+interface CampaignStatusRow {
+  status: string;
+}
+
+export interface MarkFalsePositiveResult {
+  campaignId: string;
+  status: 'false_positive';
+  auditEventId: string;
+}
+
+/**
+ * D-04 guard: rejects with RemediationConflictError when any
+ * approved/completed remediation exists for the campaign — a campaign can
+ * never be both remediated and false-positive. Otherwise sets
+ * campaigns.status='false_positive' and writes one atomic audit row
+ * recording the previous status and the optional reason. False-positive
+ * reversibility is out of scope (CONTEXT.md Deferred Ideas) — no un-mark
+ * path exists.
+ */
+export async function markCampaignFalsePositive(
+  campaignId: string,
+  actor: string | null,
+  reason?: string
+): Promise<MarkFalsePositiveResult> {
+  return postgresClient.transaction(async (client) => {
+    const guardRes = await client.query<{ id: string }>(
+      `SELECT id FROM remediation_actions
+        WHERE campaign_id = $1 AND status IN ('approved', 'completed')
+        FOR UPDATE
+        LIMIT 1`,
+      [campaignId]
+    );
+    if (guardRes.rows.length > 0) {
+      throw new RemediationConflictError(
+        'Cannot mark false positive: campaign already has approved or completed remediation'
+      );
+    }
+
+    const campaignRes = await client.query<CampaignStatusRow>(
+      `SELECT status FROM campaigns WHERE id = $1`,
+      [campaignId]
+    );
+    const campaign = campaignRes.rows[0];
+    if (!campaign) {
+      throw new RemediationValidationError('Campaign not found');
+    }
+    const previousStatus = campaign.status;
+
+    await client.query(
+      `UPDATE campaigns SET status = 'false_positive', updated_at = NOW() WHERE id = $1`,
+      [campaignId]
+    );
+
+    const auditEventId = await writeAuditEvent(
+      {
+        campaignId,
+        actor,
+        eventType: 'campaign_marked_false_positive',
+        payload: { previousStatus, reason: reason ?? null },
+      },
+      client
+    );
+
+    return { campaignId, status: 'false_positive', auditEventId };
+  });
+}