Background: Testicular adrenal rest tumors (TART) can cause infertility in congenital adrenal hyperplasia (CAH) males. Aims: To determine TART prevalence in patients with CAH due to 21-hydroxylase deficiency (21-OHD) and evaluate possible factors associated with its development. Methods: This is a descriptive and analytical cross-sectional study evaluating males with the classical form of 21-OHD through testicular ultrasonography and serum inhibin B dosages. Data on prescribed glucocorticoid dose and serum levels of 17- hydroxyprogesterone (17-OHP), androstenedione (Andro), ACTH, renin, and LH were obtained from medical records. Results: Thirty-eight males were evaluated. The mean age on ultrasonography was 15.2 ± 6.7 (3–27) years. Nine patients (23.7%) had TART, 4 of them were prepubertal and the youngest was 5 years old. No association was found between TART and 21-OHD phenotype, glucocorticoid dose, or 17-OHP, ACTH, LH, renin, and inhibin B levels measured in the 6 preceding years. However, 50% of the patients who presented increased Andro 2 years prior to the evaluation had TART (p = 0.018, OR = 8.00 [95% CI: 1.42–44.92]), whereas in the normal Andro group only 16.7% had tumors. Conclusion: This study showed that TART can occur in prepubertal patients and that disease control could be a factor associated with its development. Therefore, we suggest investigating TART development early in childhood, mainly in poorly controlled 21-OHD patients.