New parents' sexual frequency and desire fluctuate throughout the transition to parenthood (i.e., the first year after childbirth). Poorer infant sleep and parental sleep are each associated with lower sexual frequency and desire in cross-sectional research. According to the theory, infant sleep might shape new parents' sexual frequency and desire in so far as it disrupts parental sleep, though this pathway has yet to be examined. We examined the role of parental sleep in the indirect pathway between infant sleep and sexual frequency and sexual desire in couples, both within and between-person, during the first-year postpartum. In a dyadic longitudinal study, 203 first-time mothers and their partners reported on infant sleep quality, parental sleep, sexual frequency, and sexual desire at 3-, 6-, 9-, and 12-months postpartum. Poorer infant sleep was associated with mothers' (within-couple) and partners' (between-couple) poorer sleep and, in turn, lower sexual frequency for the couple. For both mothers (within-person) and partners (between-person), poorer infant sleep was associated with their own lower sexual desire through poorer parental sleep via the indirect pathway. Ongoing assessment of infant sleep and parental sleep may reveal opportunities to mitigate the negative effects of poor sleep on new parents' sexual relationships. (PsycInfo Database Record (c) 2023 APA, all rights reserved).