<p>Testicular metastasis from gastric adenocarcinoma is rare, and serial ultrasonographic evolution of this process has seldom been documented. We report a 79-year-old man with gastric adenocarcinoma who presented with progressive left-dominant scrotal enlargement. Initial scrotal ultrasonography showed bilateral hydroceles, greater on the left, with septation but no definite intratesticular lesion. One month later, repeat ultrasonography demonstrated heterogeneous left testicular echotexture with multiple small solid nodules and mildly increased vascularity, indicating interval parenchymal involvement. Pelvic magnetic resonance imaging showed bilateral suspicious testicular and epididymal abnormalities with septal enhancement and diffusion restriction. Histopathology confirmed metastatic adenocarcinoma involving the left testis and left tunica vaginalis, with CDX-2 and SATB2 positivity supporting gastrointestinal differentiation. Biopsy of the right tunica vaginalis showed hydrocele without evidence of malignancy; therefore, the contralateral abnormalities remained radiologically suspicious but were not pathologically confirmed. Follow-up ultrasonography demonstrated destructive progression of the left testis and epididymis with chaotic vascularity. This case highlights the value of serial scrotal ultrasonography for recognizing interval progression from apparently benign hydrocele-dominant findings to metastatic testicular-epididymal involvement. In older patients with known extratesticular malignancy, evolving scrotal swelling should prompt careful reassessment of testicular parenchyma and vascularity on follow-up ultrasound.</p>