肾积水（Hydronephrosis），又称水肾，是肾盂肿胀、扩大的现象，最常见的原因是尿液排泄受到阻塞所致。

由于尿流阻塞可以是急性或慢性，肾积水的症状也就有轻重不同的表现。有些单侧肾积水的患者完全没有症状。

Hydronephrosis — literally "water inside the kidney" — refers to distension and dilation of the renal pelvis and calyces, usually caused by obstruction of the free flow of urine from the kidney. Untreated, it leads to progressive atrophy of the kidney. One or both kidneys may be affected. In cases of hydroureteronephrosis, there is distention of both the ureter and the renal pelvis and calices.