OBJECTIVE Weight gain occurs during marriage, yet obesity treatment is focused on individuals. Outcomes may be improved by targeting joint weight loss and the interpersonal milieu that fosters spousal interdependence. Self-determination theory (SDT) posits that autonomy-supportive environments (e.g., promote meaningful choice, minimize control) produce better health outcomes. This trial tested an SDT-informed weight-loss intervention intended to facilitate autonomy support in couples. METHODS Sixty-four couples were randomized to standard behavioral weight loss (BWL) that couples attended together or to a SDT-informed weight-loss intervention (SDT-WL) that aimed to bolster autonomy support (AS). Groups met weekly for 6 months with assessments at 0, 3, 6, and 12 months. RESULTS Percent weight loss at 6 and 12 months was 10.4% ± 6.5% and 9.2% ± 8.2%. No differences were observed between the BWL and SDT-WL conditions in percent weight loss or changes in AS. Across conditions, higher baseline AS predicted greater weight loss at 6 and 12 months (ps <.001). Increases in AS over time predicted greater weight loss at 6 and 12 months (ps ≤ .02). Post hoc moderation analysis indicated that only participants with low (but not high) baseline AS achieved greater gains in AS at 12 months in SDT-WL than in the BWL conditions (p < .02). CONCLUSIONS Although no differences were found between conditions on weight loss or changes in autonomy support behavior, autonomy support from one's spouse predicted weight loss in both couples-based weight-loss approaches. For couples with low levels of AS, an SDT-informed approach was effective at increasing this desirable interpersonal behavior. (PsycINFO Database Record (c) 2020 APA, all rights reserved).