Low-fat plant-based diets cause weight loss in clinical trials. However, many foods are highly processed, raising the question as to their effect on body weight. This secondary analysis assessed the associations between changes in processed food intake and weight loss in 244 overweight adults randomly assigned to a vegan (n = 122) or control group (n = 122) for 16 weeks. Three-day dietary records were analyzed using the NOVA system, which categorizes foods from 1 to 4, based on degree of processing. A repeated measure ANOVA, Pearson correlations, and a multivariate regression model were used for statistical analysis. The consumption of animal foods in categories 1-4 decreased in the vegan group, compared with the control group. Body weight decreased in the vegan group (treatment effect - 5.9 kg [95% CI -6.7 to -5.0]; Gxt, p < 0.001). Changes in consumption of animal foods in categories 1-4 were positively associated with changes in body weight: r = + 0.34; p < 0.001 for category 1; r = + 0.18; p = 0.008 for category 2; r = + 0.17; p = 0.01 for category 3; and r = + 0.22; p = 0.001 for category 4. In no NOVA category was the consumption of plant-based processed foods positively and significantly associated with weight gain. The top three independent predictors of weight loss were reduced intakes of processed, unprocessed or minimally processed, and ultra-processed animal foods. These findings suggest that replacing animal products with plant-based foods may be an effective weight-loss strategy, even when processed plant-based foods are included.