This study evaluated the optimal DL-methionyl-DL-methionine (AQUAVI® Met-Met) supplementation for the red swamp crayfish Procambarus clarkii (initial body weight: 6.45 ± 0.12 g) fed plant-based diets. Seven diets were formulated, including a positive control with 50.0 g kg−1 fishmeal (PC), a negative control with 77.6 g kg−1 peanut meal replacing 50.0 g kg−1 fishmeal (NC), and five experimental diets supplemented with 0.25, 0.50, 1.00, 2.00, and 4.00 g kg−1 Met-Met. The analyzed Met-Met levels of the seven diets were 0.06 (PC), 0.10 (NC), 0.27 (M0.27), 0.46 (M0.46), 1.00 (M1.00), 1.99 (M1.99), and 4.10 g kg−1 (M4.10), respectively. After a 6-week feeding, crayfish fed the NC diet had lower growth performance, protein deposition, digestive enzyme activities, and antioxidant capacity than those fed the PC diet (P < 0.05). With the increasing Met-Met level to 1.00 g kg−1 in NC diet, the weight gain rate; specific growth rate; protein deposition rate; the activities of protease, lipase, and amylase in hepatopancreas and intestine; total antioxidant capacity; catalase and superoxide dismutase activities; and S-adenosyl homocysteine contents showed significant improvement (P < 0.05). This means the supplementation of 1.00 g kg−1 Met-Met in the non-fishmeal diet could alleviate the negative effects caused by all-plant protein formulation. Based on the broken-line regression analysis of weight gain rate and protein deposition rate in red swamp crayfish, the optimal levels of Met-Met in an all-plant protein diet were 0.90 and 0.95 g kg−1, and the levels of the total sulfur amino acid were 11.61 and 11.67 g kg−1, respectively.