The objectives of this study were: (a) to analyze the relative age effect (RAE) in the players participating in the Spanish Championship of Minibasket during the last decade, and (b) to determine a possible relationship between RAE and the final classification of the analyzed teams. All players (n = 2268) from the 19 participating teams during the years 2009-2018 were analyzed. We subdivided the players according to their date of birth into four quartiles and grouped the top 10 teams of the ranking into three different categories. The results showed RAE during the past decade (χ2 = 380.57, P = .000). In addition, the top three teams included a greater number of players born in the first half of the year (83.40%, χ2 = 183.15, P = .000). In conclusion, RAE could affect the U12- basketball performance.