Heeding recent calls to capture dynamic variability of physical activity (PA) motivation within a self-determination theory framework, this study examined the extent to which psychological needs satisfaction in PA predicted subsequent PA, disaggregating within-person and between-persons data. University students (N = 89) wore an ActiGraph GT3X accelerometer for 6 days and reported basic psychological needs satisfaction daily. Multilevel models examined whether competence, autonomy, and relatedness for the previous day’s PA (>2,020 counts per minute) predicted the following day’s minutes of PA (>2,020 counts per minute), controlling for previous-day PA. Participants who, on average, reported greater feelings of autonomy and competence tended to engage in more minutes of PA the following day. When participants reported feeling greater relatedness than what was typical for them, they tended to engage in more PA the following day. Psychological needs vary day to day, but how and to what extent they predict PA depends on the specific need.