Physical activity (PA) is critical for weight loss maintenance (WLM) success. Clarifying mechanisms behind PA engagement may suggest new WLM intervention targets. This study examined an application of temporal self-regulation theory (TST) to enhance our understanding of PA during WLM. Participants (n = 214) who achieved a ≥ 5% weight loss during a 4-month weight loss intervention were recruited into an 18-month WLM trial. TST constructs (i.e. PA beliefs, intention, behavioral prepotency, self-regulatory capacity) were measured via self-report measures. PA was subsequently assessed over a 7-day period with waist-worn Actigraph GT9X. Robust linear regression models and generalized linear mixed models tested the association between PA beliefs and intention, and the associations between intention, behavioral prepotency, self-regulatory capacity and device-measured PA at baseline and 18-months. Short-term positive beliefs were associated with intention at baseline and 18-months, whereas short-term negative beliefs were associated with intention at 18-months only. Intention was associated with moderate/vigorous PA (MVPA) minutes and bouted MVPA at baseline and 18-months. The intention by self-regulatory capacity interaction was significant at baseline. Findings lend some support for the use of TST for understanding PA and suggest that short-term beliefs about PA may represent a meaningful target for intervention.