Introduction: This large mixed-method study examines the prevalence of reported positive and negative weight-related comments from significant others and ways in which they are associated with weight-control and muscle-enhancing behaviors and emotional well-being in young adults. Method: As part of Project EAT (Eating and Activity in Teens and Young Adults)–IV, survey data were provided by 1,436 young adult men and women with a significant other in 2015–2016 (mean age = 31.1 years; 58.6% women). Independent variables included the reported frequency of receiving positive or negative comments about body shape or size; written examples of positive comments were qualitatively coded for subtypes. Analysis of covariance, adjusting for body mass index, tested associations between comments and weight-control behaviors, muscle-enhancing behaviors, and various measures of emotional well-being (i.e., body satisfaction, self-esteem, and depressive symptoms). Results: About twice as many participants received positive comments from their significant others compared to negative comments (75% vs. 36%). Receiving positive and/or negative comments was related to body satisfaction, self-esteem, and depressive symptoms but was largely unrelated to weight-control and muscle-enhancing behaviors. In almost all cases, receiving no comments was associated with significantly better well-being than was receiving only negative comments. There were few differences in weight-control or muscle-enhancing behaviors or emotional well-being across subtypes of positive comments. Discussion: Reporting the receipt of negative comments from significant others was associated with poorer emotional well-being than was receiving positive comments or no weight-related comments at all. Therapists and other health professionals working with couples should consider advising young adults on the apparent advantages of providing positive feedback regarding weight and shape or abstaining from commenting altogether.