Abstract. The importance of explicit examples of Lagrangian submanifolds of moduli spaces is revealed by papers such as [9,25]: given a 3-manifold M with boundary ∂M = Σ, Dostoglou and Salamon use such examples to obtain a proof of the Atiyah-Floer conjecture relating the symplectic Floer homology of the representation space Hom(π 1 (Σ = ∂M ), U )/U (associated to an explicit pair of Lagrangian submanifolds of this representation space) and the instanton homology of the 3-manifold M . In the present paper, we construct a Lagrangian submanifold of the space of representations M g,l := Hom C (π g,l , U )/U of the fundamental group π g,l of a punctured Riemann surface Σ g,l into an arbitrary compact connected Lie group U . This Lagrangian submanifold is obtained as the fixed-point set of an anti-symplectic involutionβ defined on M g,l . We show that the involutionβ is induced by a form-reversing involution β defined on the quasi-Hamiltonian space (The fact thatβ has a non-empty fixed-point set is a consequence of the real convexity theorem for group-valued momentum maps proved in [28]. The notion of decomposable representation provides a geometric interpretation of the Lagrangian submanifold thus obtained.