diff --git a/internal/contract/customer_auth_contract.go b/internal/contract/customer_auth_contract.go
index b7501f7..56d28dc 100644
--- a/internal/contract/customer_auth_contract.go
+++ b/internal/contract/customer_auth_contract.go
@@ -17,8 +17,9 @@ type RegisterStartRequest struct {
 	Name        string `json:"name" binding:"required"`
 	BirthDate   string `json:"birth_date" binding:"required"`
 	// The organization (brand) the customer registers with. A customer belongs to one
-	// organization; the app sends the one it is built for.
-	OrganizationID string `json:"organization_id" binding:"required"`
+	// organization. Optional: when it is left out and the database has exactly one
+	// organization, the customer joins that one.
+	OrganizationID string `json:"organization_id,omitempty"`
 }
 
 type RegisterVerifyOtpRequest struct {
diff --git a/internal/processor/customer_auth_processor.go b/internal/processor/customer_auth_processor.go
index 57356b0..02b59f9 100644
--- a/internal/processor/customer_auth_processor.go
+++ b/internal/processor/customer_auth_processor.go
@@ -143,19 +143,12 @@ func (p *customerAuthProcessor) StartRegistration(ctx context.Context, req *cont
 		return nil, fmt.Errorf("phone number already registered")
 	}
 
-	// The customer joins the organization the app is built for. Check it exists now,
-	// before an OTP is sent, rather than failing on a foreign key at the last step.
-	organizationID, err := uuid.Parse(strings.TrimSpace(req.OrganizationID))
-	if err != nil {
-		return nil, fmt.Errorf("organization_id must be a valid UUID")
-	}
-	orgExists, err := p.customerAuthRepo.OrganizationExists(ctx, organizationID)
+	// Resolve the organization before an OTP is sent, rather than failing on a foreign
+	// key at the last step.
+	organizationID, err := p.registrationOrganization(ctx, req.OrganizationID)
 	if err != nil {
 		return nil, err
 	}
-	if !orgExists {
-		return nil, fmt.Errorf("organization not found")
-	}
 
 	// Generate registration token and create OTP session
 	registrationToken := uuid.New().String()
@@ -458,3 +451,37 @@ func (p *customerAuthProcessor) ResendOtp(ctx context.Context, req *contract.Res
 }
 
 // Helper functions - OTP generation is now handled by OtpProcessor
+
+// registrationOrganization is the organization a new customer joins: the one the app
+// sent, which must exist, or, when the app sent none, the only organization there is.
+// With several organizations and none sent there is no way to choose, so it refuses.
+func (p *customerAuthProcessor) registrationOrganization(ctx context.Context, requested string) (uuid.UUID, error) {
+	requested = strings.TrimSpace(requested)
+	if requested != "" {
+		id, err := uuid.Parse(requested)
+		if err != nil {
+			return uuid.Nil, fmt.Errorf("organization_id must be a valid UUID")
+		}
+		exists, err := p.customerAuthRepo.OrganizationExists(ctx, id)
+		if err != nil {
+			return uuid.Nil, err
+		}
+		if !exists {
+			return uuid.Nil, fmt.Errorf("organization not found")
+		}
+		return id, nil
+	}
+
+	ids, err := p.customerAuthRepo.OrganizationIDs(ctx, 2)
+	if err != nil {
+		return uuid.Nil, err
+	}
+	switch len(ids) {
+	case 1:
+		return ids[0], nil
+	case 0:
+		return uuid.Nil, fmt.Errorf("no organization exists to register customers into")
+	default:
+		return uuid.Nil, fmt.Errorf("organization_id is required: there is more than one organization")
+	}
+}
diff --git a/internal/repository/customer_auth_repository.go b/internal/repository/customer_auth_repository.go
index 58fd1f6..56319d9 100644
--- a/internal/repository/customer_auth_repository.go
+++ b/internal/repository/customer_auth_repository.go
@@ -19,6 +19,8 @@ type CustomerAuthRepository interface {
 	SetCustomerPassword(ctx context.Context, customerID string, passwordHash string) error
 	// OrganizationExists reports whether an organization with this id exists.
 	OrganizationExists(ctx context.Context, organizationID uuid.UUID) (bool, error)
+	// OrganizationIDs returns up to limit organization ids.
+	OrganizationIDs(ctx context.Context, limit int) ([]uuid.UUID, error)
 }
 
 type customerAuthRepository struct {
@@ -90,3 +92,12 @@ func (r *customerAuthRepository) OrganizationExists(ctx context.Context, organiz
 	}
 	return count > 0, nil
 }
+
+func (r *customerAuthRepository) OrganizationIDs(ctx context.Context, limit int) ([]uuid.UUID, error) {
+	var ids []uuid.UUID
+	err := r.db.WithContext(ctx).Table("organizations").Order("created_at").Limit(limit).Pluck("id", &ids).Error
+	if err != nil {
+		return nil, fmt.Errorf("failed to list organizations: %w", err)
+	}
+	return ids, nil
+}
diff --git a/internal/validator/customer_auth_validator.go b/internal/validator/customer_auth_validator.go
index 51df18f..a62f459 100644
--- a/internal/validator/customer_auth_validator.go
+++ b/internal/validator/customer_auth_validator.go
@@ -71,8 +71,10 @@ func (v *CustomerAuthValidatorImpl) ValidateRegisterStartRequest(req *contract.R
 	}
 
 	// Validate organization
-	if _, err := uuid.Parse(strings.TrimSpace(req.OrganizationID)); err != nil {
-		return errors.New("organization_id must be a valid UUID"), constants.ValidationErrorCode
+	if orgID := strings.TrimSpace(req.OrganizationID); orgID != "" {
+		if _, err := uuid.Parse(orgID); err != nil {
+			return errors.New("organization_id must be a valid UUID"), constants.ValidationErrorCode
+		}
 	}
 
 	// Validate birth date