Background: Splenogonadal fusion (SGF) is a rare congenital malformation that occurs during embryonic development. SGF typically presents as a left-sided scrotal swelling, left inguinal hernia, scrotal mass, or cryptorchidism. Therefore, it is easily misdiagnosed, and unnecessary orchiectomy may occur. This study aimed to report a rare case of SGF. Case presentation: A 5-month-old male child presented with a history of obvious left scrotal swelling for 1 month, which progressively worsened 10 h before the hospital visit. The ultrasound examination exhibited a solid mass in the left scrotum, with echo quite similar to that in the testicle and went up into the abdominal cavity through an identical echogenic band structure. After surgical resection, the pathological examination confirmed that the submitted tissue was spleen tissue with extensive bleeding. Conclusion: Ultrasound can provide important information regarding the diagnosis of SGF. The possibility of SGF should be considered for further differential diagnosis in the case of similar patients.