Romantic partner undermining is a type of negative social influence that has the potential to make weight loss more challenging and consists of two components: (1) undermining behaviors, or overt romantic partner behaviors including criticism/complaint of weight control or interference behaviors (e.g., offering up high calorie foods), and (2) undermining attitudes, including negative beliefs that a romantic partner has about their significant other's weight loss attempts. The present study was the first to the authors' knowledge to investigate romantic partner undermining attitudes and behaviors reported by participants enrolled in in a behavioral weight loss (BWL) intervention, a first line treatment for weight loss. The study also examined whether partner undermining attitudes and behaviors differed by relationship quality and between those who did and did not choose their romantic partner to serve in a support role in the intervention. Participants (N = 128) enrolled in a group-based BWL intervention reported on perceived romantic partner undermining attitudes and behaviors at baseline and 3 months (end of the intensive period of BWL intervention). Results showed that participants perceived minimal romantic partner undermining at baseline (attitudes M = 2.2 ± 0.7, behaviors M = 2.3 ± 1.2) and 3 months (attitudes M = 2.0 ± 0.6, behaviors M = 1.9 ± 1.1). However, participants reported a significant reduction in total undermining attitudes (M change = - 0.2 points) and behaviors (M change = - 0.4 points) from baseline to 3 months (ps <.05), highlighting the potential of group-based BWL interventions to mitigate romantic partner undermining without explicit content focused on undermining. Future research should examine romantic partner undermining across time in varying weight control and relationship conditions and measure undermining at the daily or weekly level to reduce recall bias and improve ecological validity.