In eusocial Hymenoptera, haplodiploidy and polyandry may facilitate selection for hybridization. Interspecific hybridization is widespread in ants and can lead to hybrid inviability as well as the formation of new species through hybrid speciation. However, in ants, polyandry is uncommon. By analyzing microsatellite markers on 15 ant workers per colony, we show that the mating system of 28 pure colonies of Tetramorium immigrans, 15 pure colonies of Tetramorium caespitum, and 27 hybrid colonies is a monogyne/polyandrous mating system, with a higher mating rate in T. caespitum (mean = 2.4 males vs. 1.7 in T. immigrans). Hybrid queens, but no hybrid fathers, were deduced from workers' genotypes, in accordance with Haldane's rule extended to haplodiploid organisms, which states that the haploid sex should more often be sterile or inviable. In five colonies, hybridization and multiple mating allowed the simultaneous production of both hybrid and nonhybrid offspring. Although rare, these situations hinted at asymmetrical, larger contributions of T. immigrans vs. T. caespitum males to offspring production. Together, these findings point toward a complex and dynamic mating system in T. immigrans and T. caespitum, and contribute to better understand interspecific hybridization mechanisms and their consequences on genetic and taxonomic diversity. The study of polyandry within a hybrid zone is unprecedented and opens new opportunities to better understand interspecific hybridization mechanisms and their short- to long-term consequences.