Background: Food-based dietary patterns emphasizing plant protein that were evaluated in the Dietary Approaches to Stop Hypertension (DASH) and OmniHeart trials are recommended for the treatment of metabolic syndrome (MetS). However, the contribution of plant protein to total protein in these diets is proportionally less than that of animal protein.Objective: This study compared 3 diets varying in type (animal compared with plant) and amount of protein on MetS criteria.Design: Sixty-two overweight adults with MetS consumed a healthy American diet for 2 wk before being randomly allocated to either a modified DASH diet rich in plant protein (18% protein, two-thirds plant sources, n = 9 males, 12 females), a modified DASH diet rich in animal protein (Beef in an Optimal Lean Diet: 18.4% protein, two-thirds animal sources, n = 9 males, 11 females), or a moderate-protein diet (Beef in an Optimal Lean Diet Plus Protein: 27% protein, two-thirds animal sources, n = 10 males, 11 females). Diets were compared across 3 phases of energy balance: 5 wk of controlled (all foods provided) weight maintenance (WM), 6 wk of controlled weight loss (minimum 500-kcal/d deficit) including exercise (WL), and 12 wk of prescribed, free-living weight loss (FL). The primary endpoint was change in MetS criteria.Results: All groups achieved ∼5% weight loss at the end of the WL phase and maintained it through FL, with no between-diet differences (WM compared with WL, FL, P < 0.0001; between diets, P = NS). All MetS criteria decreased independent of diet composition (main effect of phase, P < 0.01; between diets, P = NS). After WM, all groups had a MetS prevalence of 80–90% [healthy American diet (HAD) compared with WM, P = NS], which decreased to 50–60% after WL and was maintained through FL (HAD, WM vs WL, FL, P < 0.01).Conclusions: Weight loss was the primary modifier of MetS resolution in our study population regardless of protein source or amount. Our findings demonstrate that heart-healthy weight-loss dietary patterns that emphasize either animal or plant protein improve MetS criteria similarly. This study was registered at clinicaltrials.gov as NCT00937638.