Background <p>Thrombosis of varicocele veins is extremely rare and may mimic other causes of acute scrotal pain. Spontaneous thrombosis occurring in adults without trauma, surgery, or known coagulation disorders is seldom reported.</p> Case presentation <p>A 54-year-old male with type 2 diabetes mellitus presented with acute left testicular pain radiating to the left inguinal region. Physical examination revealed a soft left testis with a palpable left scrotal sac cord-like swelling, while the right testis and scrotum were normal. Color Doppler ultrasonography, performed in supine and erect positions using a SonoAce X8 (Medison, Korea) ultrasound system with a 5–12&#xa0;MHz superficial probe, demonstrated a grade III left varicocele with dilated refluxing veins and echogenic intraluminal thrombus within some scrotal veins. There was no history of trauma, heavy activity, surgery, or coagulation disorder. A diagnosis of spontaneous thrombosis of a left-sided varicocele was established.</p> Conclusions <p>This case highlights the importance of differentiating primary from secondary varicoceles, excluding tumor thrombus, and recognizing the role of diabetes-induced endothelial dysfunction as a potential risk factor. Awareness of this rare entity can guide accurate diagnosis and management, preventing unnecessary intervention.</p>