Kidney dialysis (from Greek διάλυσις, dialysis, 'dissolution'; from διά, dia, 'through', and λύσις, lysis, 'loosening or splitting') is the process of removing excess water, solutes, and toxins from the blood in people whose kidneys can no longer perform these functions naturally. This is referred to as renal replacement therapy. The first successful dialysis was performed in 1943.

Dialysis may need to be initiated when there is a sudden rapid loss of kidney function, known as acute kidney injury (previously called acute renal failure), or when a gradual decline in kidney function, chronic kidney failure, reaches stage 5. Stage 5 chronic renal failure is reached when the glomerular filtration rate is 10–15% of the normal, creatinine clearance is less than 10 mL per minute, and uremia is present.

Dialysis is used as a temporary measure in either acute kidney injury or in those awaiting kidney transplant and as a permanent measure in those for whom a transplant is not indicated or not possible.

In West European countries, Australia, Canada, the United Kingdom, and the United States, dialysis is paid for by the government for those who are eligible.