In polygynous species, males typically mate with more than one female, and male investment in any one mating should decrease if securing that mating reduces future mating opportunities. In contrast, monogynous males mate only once, so they should invest maximally in that single mating. In two monogynous, sexually cannibalistic Latrodectus spider species (L. geometricus, L. hasselti), males can mate and fertilize immature females. This recently described tactic is time-consuming because males must wait days for females to be capable of mating. It is also risky since immature females approached too early may kill males outright. However, if males typically find only one female in their lifetime, increasing the opportunity to mate may be worth these costs. We investigated whether this tactic is also practiced by a polygynous congener, L. hesperus, in which males typically mate more than once and may avoid time-consuming, risky encounters. In laboratory trials, we showed that males copulate with immature females. Moreover, males mounted immatures more rapidly, copulated for longer, and fathered more offspring than males that mated adults females. We concluded that monogyny is not a necessary condition for immature mating to be favored as an alternative reproductive tactic and suggest that it may be common in other spider taxa.