Retrieving these forgotten encrusted double j (DJ) stents could be challenging and require multimodal urological interventions, especially in a single-functioning transplanted kidney. Only a few cases of mini-percutaneous nephrolithotomy (mini-PCNL) via ultrasonography (US) guidance in transplanted kidneys have been published. We report a 47-year-old man; a case of a transplanted kidney, with a complaint of abdominal pain and dysuria for two weeks. The patient had a forgotten DJ stent for more than a year due to a lack of post-procedural follow-up information. An abdominal computed tomography scan showed a 20 x 13mm stone in the proximal tip of the DJ stent. Ultrasonography-guided mini-PCNL with a 19-fr rigid nephroscope was performed without complications. In conclusion, we emphasize the importance of patient education regarding the indwelling DJ stent. This case also provides that if an experienced urologist performs it, the US-guided mini-PCNL is safe and effective in transplanted kidney patients.