Congenital mesoblastic nephroma (CMN) is the most common renal tumor of early infancy, and infants with renal masses suspicious for CMN often undergo upfront radical nephrectomy with proximal ureterectomy. Infective mimics, including renal abscess, acute lobar nephronia, and xanthogranulomatous pyelonephritis, are considerably less common, particularly when typical clinical signs of infection are absent or muted. We report the case of a 2-month-old boy with a painless, progressively enlarging left abdominal mass first noted at 20 days of age, accompanied by intermittent low-grade fever and constipation. Contrast-enhanced computed tomography showed an ill-defined, heterogeneously enhancing left renal mass with perinephric extension and regional lymphadenopathy, raising suspicion for CMN. The infant underwent radical left nephroureterectomy with splenic flexure resection, the latter of which was required because of dense inflammatory adhesions. Histopathological examination revealed a chronic suppurative renal abscess with palisading histiocytes and multinucleated giant cells, with no evidence of malignancy, fungal organisms, or mycobacteria. Postoperative recovery was uneventful, and the child remains well during follow-up. This case illustrates a rare but clinically important diagnostic pitfall: infective renal masses can masquerade as malignant renal tumors in early infancy, particularly when systemic inflammatory features are subtle.