Malaria parasite fertilisation occurs within the Anopheles mosquito midgut. Interventions that inhibit parasite fertilisation prevent ongoing transmission and are important for malaria elimination efforts. Pfs48/45 and Pfs230 are two leading transmission-blocking vaccine candidates. Both proteins form a complex on the surface of sexual stage parasites and are essential for male fertility. Here we have identified nanobodies against Pfs48/45 that recognise gametocytes and have strong transmission-reducing activity. The crystal structure of our most potent nanobody in complex with Pfs48/45 reveals it binds a distinct epitope to TB31F, a leading transmission-blocking monoclonal antibody. In addition, we generated bispecific nanobodies that can target both Pfs48/45 and Pfs230 simultaneously and are fused to a human Fc domain. Our results show that these bispecific nanobodies recognise both Pfs48/45 and Pfs230 and reduce malaria parasite fertilisation in Anopheles stephensi. These results demonstrate the potential of nanobodies as a versatile antibody format that can reduce malaria transmission.