The connective constant μ(G)μ(G) of a graph GG is the asymptotic growth rate of the number σnσn of self-avoiding walks of length nn in GG from a given vertex. We prove a formula for the connective constant for free products of quasi-transitive graphs and show that σn∼AGμ(G)nσn∼AGμ(G)n for some constant AGAG that depends on GG. In the case of products of finite graphs μ(G)μ(G) can be calculated explicitly and is shown to be an algebraic number.