Why do people have better sexual experiences on certain days but not others? In this study, we used self-determination theory (SDT) to examine whether sexual motives that are autonomous (i.e., genuinely self-endorsed) as opposed to controlled (i.e., pressured) were associated with variations in daily sexual well-being. We also sought to identify circumstances associated with changes in the quality of sexual motivation by considering the extent to which daily interactions with a partner satisfied the basic psychological needs for autonomy, competence, and relatedness. In a 21-day study of daily experiences ( N = 113), findings from multilevel analyses suggested that on days when sexual motives were more autonomous and less controlled, people experienced higher sexual well-being (i.e., higher sexual satisfaction, more positive sexual affect, and less negative sexual affect). Furthermore, on days when interactions with the partner were more positive, as evidenced by better needs satisfaction, sexual motives were more autonomous and this, in turn, was associated with higher sexual well-being. These associations held above the contributions of gender, relationship length, frequency of sexual activities, and relational satisfaction. These findings demonstrate the usefulness of SDT as a framework for the motivational underpinnings of sexual well-being.