We show a Poisson formula for bounded harmonic functions Φ on the Sierpiński gasket S. We construct a boundary (Σ̃, ν̃), a measurable action of a semigroup W on Σ̃ and a map Γ: W → S, such that for every bounded harmonic function Φ on S Φ(Γ(w)) = ∫Σ̃ Ψ(wξ)ν̃(dξ), where Ψ : Σ̃ → ℝ is some bounded measurable function.