Little is known about how couples' social support facilitates the pursuit of important goals in daily life. Using an interpersonal perspective, we examined the effects of support provision and receipt on same-day physical activity, and studied the role of partners' joint engagement in activities. One hundred nineteen heterosexual couples reported on target persons' received and partners' provided support across 28 diary days, yielding 2,854 valid days. A dyadic report on couples' joint engagement was obtained from a subset of 88 couples. Target persons' daily activity was objectively assessed via accelerometers. On days with high versus low levels of provided support, target persons' activity was 25 min higher. Support receipt mediated 20% of this effect. Joint engagement accounted for around half of the effects of provided and received support. Support provision is uniquely linked to goal implementation in everyday life. Joint engagement in activities may be one explanation for how support is facilitated.