Malaria parasites produce male and female life cycle stages (gametocytes) that must fertilize to achieve successful colonization of the mosquito. Gametocyte sex ratios have been shown to be under strong selection pressure both as an adaptive response to a worsening blood environment for transmission and according to the number of co-infecting clones in the vertebrate. Evidence for an impact of sex ratio on the transmission success of Plasmodium falciparum has, however, been more controversial. Theoretical models of fertilization predict that increasingly male sex ratios will be favoured at low gametocyte densities to ensure fertilization. Here, we analyse in vitro transmission studies of P. falciparum to Anopheles gambiae mosquitoes and test this prediction. We find that there is a discernible effect of sex ratio on transmission but which is dependent upon the gametocyte density. While increasingly male sex ratios do give higher transmission success at low gametocyte densities, they reduce success at higher densities. This therefore provides empirical confirmation that sex ratio has an immediate impact on transmission success and that it is density-dependent. Identifying the signals used by the parasite to alter its sex ratio is essential to determine the success of transmission-blocking vaccines that aim to impede the fertilization process.