diff --git a/docs/adr/README.md b/docs/adr/README.md
index 5568fcc..b8b4e9f 100644
--- a/docs/adr/README.md
+++ b/docs/adr/README.md
@@ -18,6 +18,7 @@ the documentation reconciliation. They are historical context, not current specs
 | [backend-deployment-constraints.md](./backend-deployment-constraints.md) | Deployment requirements (Solaria, Docker) |
 | [mobile-health-frameworks-data.md](./mobile-health-frameworks-data.md) | HealthKit / Health Connect data-type reference (for future mobile work) |
 | [android-health-connect-data-types.md](./android-health-connect-data-types.md) | Android Health Connect data types |
+| [zero-knowledge-encryption.md](./zero-knowledge-encryption.md) | Client-side zero-knowledge encryption (AES-256-GCM, double-PBKDF2, Phase 1) |
 
 > **Note**: Where a decision diverges from the implemented code (e.g. state
 > management), the code is the source of truth and the ADR is kept only as
diff --git a/docs/adr/zero-knowledge-encryption.md b/docs/adr/zero-knowledge-encryption.md
new file mode 100644
index 0000000..2b6be96
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+++ b/docs/adr/zero-knowledge-encryption.md
@@ -0,0 +1,69 @@
+# ADR: Zero-Knowledge Encryption (Phase 1)
+
+**Status**: Implemented (Phase 1)
+**Date**: 2026-06-28
+
+## Context
+
+The original design (`docs/product/encryption.md`, `docs/adr/mongodb-schema-decision.md`)
+specified client-side zero-knowledge encryption: the server should never hold
+plaintext user data or encryption keys. Until this change, the `EncryptedField`
+type and recovery-phrase fields were aspirational scaffolding — data was always
+stored and processed as plaintext JSON.
+
+## Decision
+
+Implement **client-side zero-knowledge encryption** using AES-256-GCM via the
+browser's Web Crypto API. The server becomes a blind store: it holds opaque
+ciphertext and can never decrypt user data.
+
+### Key derivation: double PBKDF2
+
+From the user's password, the client derives two independent values:
+- **Auth secret**: `PBKDF2(password, "normogen-auth-v1", 150k iters, SHA-256)` →
+  base64, sent to the server as the "password". The server PBKDF2-hashes it
+  (as before).
+- **Encryption key**: `PBKDF2(password, "normogen-enc-v1", 150k iters, SHA-256)` →
+  AES-GCM `CryptoKey`, kept in memory only, never transmitted.
+
+The server cannot derive the encryption key because it never sees the raw
+password or the enc-domain PBKDF2 output.
+
+### What's encrypted
+
+- Medication data blobs (name, dosage, frequency, route, etc.)
+- Appointment data blobs (title, provider, date/time, etc.)
+- Profile display name
+
+### What stays plaintext (queryable)
+
+- IDs (`medicationId`, `appointmentId`, `userId`, `profileId`)
+- Medication `active` flag (top-level, filterable)
+- Appointment `status` (top-level, filterable)
+- Timestamps
+
+### In-memory key lifecycle
+
+The encryption key lives only in browser memory for the authenticated session.
+It is NOT persisted. A page reload requires re-entering the password to
+re-derive it. This is the core ZK trade-off.
+
+## Consequences
+
+### Phase 1 limitations (documented)
+- **Forgotten password = data loss.** No recovery key-wrapping yet (Phase 2).
+- **Page reload requires re-authentication** to re-derive the enc key.
+- No data migration was needed (no real user data existed).
+
+### Phase 2 (future)
+- Recovery key-wrapping (wrap the enc key under a recovery-derived key so a
+  forgotten password doesn't lose data).
+- Password-change re-wrapping (re-wrap the enc key under the new password).
+- Wrapped-DEK model (separate data-encryption key + key-encryption key).
+
+## Relationship to the original design doc
+
+`docs/product/encryption.md` describes a fuller zero-knowledge architecture
+(per-user keys, deterministic encryption for search, key rotation). This Phase 1
+implements the core property (server can't read data) with the simplest viable
+design. The fuller features remain future work.