For decades, researchers have documented significant skews in the production of male versus female offspring in many species. Because males and females are differentially susceptible to environmental challenges and also represent different fitness benefits, it may be beneficial to exert control over the offspring sex ratio when environmental conditions become challenging. Some of the most dramatic environmental challenges occur on a seasonal basis. Indeed, seasonal variation in offspring sex ratios has been documented in both mammalian and nonmammalian species. The seasonal environmental factor (or factors) that drives the skews in sex ratios is unknown; however, it is essential that such a cue be predictable and reliable and that it does not vary from year to year. We hypothesized that photoperiod, a stable cue of seasonal changes in temperature and resource availability, may underlie seasonal variation in offspring sex ratios of mammals. We predicted that short day lengths in particular, which signal impending winter conditions and related energetic demands, would stimulate an anticipatory skew in the offspring sex ratio. We used Siberian hamsters as models because they are phenotypically responsive to photoperiod but up to 60% of females continue to breed during the winter. The sexes of weanling hamsters conceived and raised in short, winter like day lengths were significantly skewed toward males. Furthermore, these skews occurred before birth; embryos collected from pregnant females maintained in short-day conditions were also significantly male biased. Thus, photoperiod functions as an effective seasonal cue, stimulating sex ratio skews toward males when day lengths are short.