Purpose: This study aimed to verify the body mass (BM) gain between individual and the judo mixed team (JMT) weigh-ins (2–7 days) and its impact on performance during the Tokyo 2020 Olympics. Methods: A total of 105 athletes composed JMTs, but only 68 competed: 39 were from lighter weight categories than JMT ones (i.e., ≤ 57 kg and ≤ 70 kg for females, and ≤ 73 kg and ≤ 90 kg for males) and allowed to gain BM up to the limit of JMT categories, whereas 29 pertained to the JMT categories and were limited by 2 kg between weigh-ins. Results: The lighter group gained more BM (4.0 ± 2.1 kg versus 1.6 ± 0.8 kg; Z = −6.52; P < 0.001; r = −0.64), a higher percentage of BM (5.7 ± 2.9% versus 2.3 ± 1.2%; Z = −6.13; P < 0.001; r = −0.60), and a higher percentage of BM per day (1.5 ± 0.9% per day versus 0.6 ± 0.3% per day; Z = −6.42; P < 0.001; r = 0.63) than those limited by the 2 kg rule. Total match time (r = 0.64), total number of matches (r = 0.67), and days between weigh-ins (r = 0.48) were positively correlated with BM gain (P < 0.05). Conclusions: BM increment was high, but correlated with more frequent and longer matches. [ABSTRACT FROM AUTHOR]