In the present paper we study the generalized Hyers-Ulam stability of the generalized trigonometric functional equationswhere S is a semigroup, σ: S −→ S is a involutive morphism, and µ: S −→ C is a multiplicative function such that µ(xσ(x)) = 1 for all x ∈ S. As an application, we establish the generalized Hyers-Ulam stability theorem on amenable monoids and when σ is an involutive automorphism of S.