<p>Engaging in sexual fantasy is commonly understood as a solitary sexual behavior, yet it is well established that people also engage in sexual fantasy during partnered sex. Despite this, little is known about how the content of people’s sexual fantasies differ across these contexts, especially for people in committed relationships. Fantasies may differ markedly in whom they focus on and how erotic and/or nurturant they are, depending on whether partners are present when these fantasies take place. In an online study, gender/sex and sexually diverse participants (<i>N</i> = 546) who had been in a committed relationship for at least six months described their most recent sexual fantasies during solitary masturbation and during sex with their committed relationship partner(s). We assessed whom they fantasized about and how erotic and nurturant their fantasies were. Fantasies during partnered sex were significantly higher in nurturant fantasy content and more likely to focus on relationship partners than fantasies during solitary masturbation. In contrast, fantasies during solitary masturbation were significantly higher in erotic fantasy content, but only if they were focused on someone other than a relationship partner. Fantasy content also differed by sexual desire, sexual satisfaction, and relationship configuration. Overall, these findings show that sexual fantasy can be context-dependent, differing across solitary and partnered environments as well as other interpersonal variables. They also highlight the importance of attending to nurturant fantasy content, not just erotic fantasy content, in understanding how sexual fantasy is related to relational and sexual wellbeing for people in relationships.</p>