Boxing is a contact sport in which the athletes compete in weight categories. The athletes use different strategies to achieve their desired weight, some of which may affect their sports performance. This systematic review assesses the scientific literature regarding the nutritional and dietetic strategies used by boxers and their effects on performance. The Physiotherapy Evidence Database (PEDro) scale was used to evaluate the quality of the articles; the maximum score was 10 points, because item 1 on the scale was not evaluated. Of a total 1549 articles obtained, only 14 remained after applying the inclusion criteria. Of these, only 2 articles scored more than 7 points on the PEDro scale and were considered to be high-quality scientific evidence. The remaining 12 articles, with scores less than 7 points, were of low quality due mainly to their small sample size (N’s for boxers range from 1–32), poor methodology, and short duration (1–12 weeks). The article included boxers of various ages, adolescents, and adults and 2 very-low-quality articles on female boxers (2 and 3 on the PEDro scale). The boxers used many unhealthy strategies to lose weight in the days prior to competition, including dehydration, energy restriction, diuretics, laxatives, and increased energy expenditure by strenuous physical activity.