diff --git a/backend/internal/db/postgres/acl_repo.go b/backend/internal/db/postgres/acl_repo.go
new file mode 100644
index 0000000..52bff64
--- /dev/null
+++ b/backend/internal/db/postgres/acl_repo.go
@@ -0,0 +1,118 @@
+package postgres
+
+import (
+	"context"
+	"errors"
+	"fmt"
+
+	"github.com/google/uuid"
+	"github.com/jackc/pgx/v5"
+	"github.com/jackc/pgx/v5/pgxpool"
+
+	"tanabata/backend/internal/domain"
+	"tanabata/backend/internal/port"
+)
+
+type permissionRow struct {
+	UserID       int16     `db:"user_id"`
+	UserName     string    `db:"user_name"`
+	ObjectTypeID int16     `db:"object_type_id"`
+	ObjectID     uuid.UUID `db:"object_id"`
+	CanView      bool      `db:"can_view"`
+	CanEdit      bool      `db:"can_edit"`
+}
+
+func toPermission(r permissionRow) domain.Permission {
+	return domain.Permission{
+		UserID:       r.UserID,
+		UserName:     r.UserName,
+		ObjectTypeID: r.ObjectTypeID,
+		ObjectID:     r.ObjectID,
+		CanView:      r.CanView,
+		CanEdit:      r.CanEdit,
+	}
+}
+
+// ACLRepo implements port.ACLRepo using PostgreSQL.
+type ACLRepo struct {
+	pool *pgxpool.Pool
+}
+
+// NewACLRepo creates an ACLRepo backed by pool.
+func NewACLRepo(pool *pgxpool.Pool) *ACLRepo {
+	return &ACLRepo{pool: pool}
+}
+
+var _ port.ACLRepo = (*ACLRepo)(nil)
+
+func (r *ACLRepo) List(ctx context.Context, objectTypeID int16, objectID uuid.UUID) ([]domain.Permission, error) {
+	const sql = `
+		SELECT p.user_id, u.name AS user_name, p.object_type_id, p.object_id,
+		       p.can_view, p.can_edit
+		FROM acl.permissions p
+		JOIN core.users u ON u.id = p.user_id
+		WHERE p.object_type_id = $1 AND p.object_id = $2
+		ORDER BY u.name`
+
+	q := connOrTx(ctx, r.pool)
+	rows, err := q.Query(ctx, sql, objectTypeID, objectID)
+	if err != nil {
+		return nil, fmt.Errorf("ACLRepo.List: %w", err)
+	}
+	collected, err := pgx.CollectRows(rows, pgx.RowToStructByName[permissionRow])
+	if err != nil {
+		return nil, fmt.Errorf("ACLRepo.List scan: %w", err)
+	}
+	perms := make([]domain.Permission, len(collected))
+	for i, row := range collected {
+		perms[i] = toPermission(row)
+	}
+	return perms, nil
+}
+
+func (r *ACLRepo) Get(ctx context.Context, userID int16, objectTypeID int16, objectID uuid.UUID) (*domain.Permission, error) {
+	const sql = `
+		SELECT p.user_id, u.name AS user_name, p.object_type_id, p.object_id,
+		       p.can_view, p.can_edit
+		FROM acl.permissions p
+		JOIN core.users u ON u.id = p.user_id
+		WHERE p.user_id = $1 AND p.object_type_id = $2 AND p.object_id = $3`
+
+	q := connOrTx(ctx, r.pool)
+	rows, err := q.Query(ctx, sql, userID, objectTypeID, objectID)
+	if err != nil {
+		return nil, fmt.Errorf("ACLRepo.Get: %w", err)
+	}
+	row, err := pgx.CollectOneRow(rows, pgx.RowToStructByName[permissionRow])
+	if err != nil {
+		if errors.Is(err, pgx.ErrNoRows) {
+			return nil, domain.ErrNotFound
+		}
+		return nil, fmt.Errorf("ACLRepo.Get scan: %w", err)
+	}
+	p := toPermission(row)
+	return &p, nil
+}
+
+func (r *ACLRepo) Set(ctx context.Context, objectTypeID int16, objectID uuid.UUID, perms []domain.Permission) error {
+	q := connOrTx(ctx, r.pool)
+
+	const del = `DELETE FROM acl.permissions WHERE object_type_id = $1 AND object_id = $2`
+	if _, err := q.Exec(ctx, del, objectTypeID, objectID); err != nil {
+		return fmt.Errorf("ACLRepo.Set delete: %w", err)
+	}
+
+	if len(perms) == 0 {
+		return nil
+	}
+
+	const ins = `
+		INSERT INTO acl.permissions (user_id, object_type_id, object_id, can_view, can_edit)
+		VALUES ($1, $2, $3, $4, $5)`
+	for _, p := range perms {
+		if _, err := q.Exec(ctx, ins, p.UserID, objectTypeID, objectID, p.CanView, p.CanEdit); err != nil {
+			return fmt.Errorf("ACLRepo.Set insert: %w", err)
+		}
+	}
+	return nil
+}
diff --git a/backend/internal/service/acl_service.go b/backend/internal/service/acl_service.go
new file mode 100644
index 0000000..ca5868f
--- /dev/null
+++ b/backend/internal/service/acl_service.go
@@ -0,0 +1,81 @@
+package service
+
+import (
+	"context"
+	"errors"
+
+	"github.com/google/uuid"
+
+	"tanabata/backend/internal/domain"
+	"tanabata/backend/internal/port"
+)
+
+// ACLService handles access control checks and permission management.
+type ACLService struct {
+	aclRepo port.ACLRepo
+}
+
+func NewACLService(aclRepo port.ACLRepo) *ACLService {
+	return &ACLService{aclRepo: aclRepo}
+}
+
+// CanView returns true if the user may view the object.
+// isAdmin, creatorID, isPublic must be populated from the object record by the caller.
+func (s *ACLService) CanView(
+	ctx context.Context,
+	userID int16, isAdmin bool,
+	creatorID int16, isPublic bool,
+	objectTypeID int16, objectID uuid.UUID,
+) (bool, error) {
+	if isAdmin {
+		return true, nil
+	}
+	if isPublic {
+		return true, nil
+	}
+	if userID == creatorID {
+		return true, nil
+	}
+	perm, err := s.aclRepo.Get(ctx, userID, objectTypeID, objectID)
+	if err != nil {
+		if errors.Is(err, domain.ErrNotFound) {
+			return false, nil
+		}
+		return false, err
+	}
+	return perm.CanView, nil
+}
+
+// CanEdit returns true if the user may edit the object.
+// is_public does not grant edit access; only admins, creators, and explicit grants.
+func (s *ACLService) CanEdit(
+	ctx context.Context,
+	userID int16, isAdmin bool,
+	creatorID int16,
+	objectTypeID int16, objectID uuid.UUID,
+) (bool, error) {
+	if isAdmin {
+		return true, nil
+	}
+	if userID == creatorID {
+		return true, nil
+	}
+	perm, err := s.aclRepo.Get(ctx, userID, objectTypeID, objectID)
+	if err != nil {
+		if errors.Is(err, domain.ErrNotFound) {
+			return false, nil
+		}
+		return false, err
+	}
+	return perm.CanEdit, nil
+}
+
+// GetPermissions returns all explicit ACL entries for an object.
+func (s *ACLService) GetPermissions(ctx context.Context, objectTypeID int16, objectID uuid.UUID) ([]domain.Permission, error) {
+	return s.aclRepo.List(ctx, objectTypeID, objectID)
+}
+
+// SetPermissions replaces all ACL entries for an object (full replace semantics).
+func (s *ACLService) SetPermissions(ctx context.Context, objectTypeID int16, objectID uuid.UUID, perms []domain.Permission) error {
+	return s.aclRepo.Set(ctx, objectTypeID, objectID, perms)
+}