There is substantial variation in the relapse frequency of Plasmodium vivax malaria, with fast relapsing strains in tropical areas, and slow relapsing strains in temperate areas with seasonal transmission. We hypothesise that much of the phenotypic diversity in P. vivax relapses arises from selection of relapse frequency to optimise transmission potential in a given environment, in a process similar to the virulence trade-off hypothesis. We develop mathematical models of P. vivax transmission and calculate the basic reproduction number R0 to investigate how transmission potential varies with relapse frequency and seasonality. In tropical zones with year round transmission, transmission potential is optimised at intermediate relapse frequencies of 2-3 months: slower relapsing strains increase the opportunity for onward transmission to mosquitoes, but also increase the risk of being outcompeted by faster relapsing strains. Seasonality is an important driver of relapse frequency for temperate strains, with the time to first relapse predicted to be 6-9 months, coinciding with the duration between seasonal transmission peaks. We predict there is a threshold degree of seasonality, below which fast relapsing tropical strains are selected for, and above which slow relapsing temperate strains dominate, providing an explanation for the observed global distribution of relapse phenotypes.