Environmental stressors can limit access to resources in numerous ways, shaping an organism's condition and fitness within and across generations. Resource constraints are especially important for sexual selection, as many components of mating behaviour, from sexual signalling to sperm production, are condition-dependent. Environmental stress can therefore be an important ecological mediator of patterns of sexual selection. In this study, we measured condition-dependent male reproductive success in the haplodiploid spider mite Tetranychus urticae. Males were placed in different host-plant environments, generating different stress levels (bean = low stress, tomato = intermediate, and tomato with cadmium = high stress) during their juvenile stage. The impact of the male juvenile environment was measured by placing males, once adult, with females from a low-stress environment. We measured several fitness traits for males, including mating success, partner fecundity, number of female offspring reaching adulthood, and offspring sex ratio. Juvenile environment did not influence adult male mating success, but males from the high stress environment suffered reduced reproductive success, producing fewer female offspring, suggesting that stressed males either pass fewer sperm or less/lower quality seminal fluid to females if they do mate. Likewise, offspring sex ratios were correlated with male stress, with sex ratios becoming less female-biased with increasing juvenile stress. The male juvenile environment in a haplodiploid species can reduce their reproductive success and prevent females mated to these males from producing optimal offspring sex ratios. Environmental stress experienced by adults can therefore impact both males and females and, through effects on sex allocation, their offspring.