Moreno-Pérez, V, Machar, R, Sanz-Rivas, D, and Del Coso, J. ACTN3 R577X genotype in professional and amateur tennis players. J Strength Cond Res 34(4): 952-956, 2020-Several investigations have concluded that the RR genotype in the ACTN3 R577X polymorphism is overrepresented in elite athletes of strength- and power-based sports when compared with nonelite populations, suggesting a positive role of this genotype on physical performance. However, no investigation has been geared to determine the distribution of this polymorphism in elite tennis players. The aim of this study was to compare the frequency distribution of the ACTN3 R577X genotype in professional and nonprofessional tennis players to determine whether this polymorphism has an association with tennis performance. A total of 128 tennis players volunteered to participate in this study. From the total, 56 were professional tennis players (16 were top 10 in Women Tennis Association/Association of Tennis Professionals [WTA/ATP] rankings, 22 were top 100 in WTA/ATP rankings, and 18 were ranked > top 100 in the WTA/ATP rankings), and the remaining 72 were categorized as nonprofessional. Chi-squared tests were used to assess differences in the distribution of RR, RX, and XX genotypes between the different performance categories. The distribution of the RR, RX, and XX genotypes was similar in professional (33.9, 48.2, and 17.9%) and nonprofessional tennis players (37.5, 40.3, and 22.2%; p = 0.650). Furthermore, the distribution of the ACTN3 R577X polymorphism was not different in top 10 (25.0, 50.0, and 25.0%), top 100 (31.8, 50.0, and 18.2%), and WTA/ATP-ranked players (44.4, 44.4, and 11.2%; p = 0.847). These results indicate that the distribution of the ACTN3 R577X genotype is similar in tennis players of very different performance levels. This outcome suggests that the ACTN3 genotype was not associated with elite tennis performance.