It only leaks when the faucet is running.

Running the faucet also sends water down the drain, so this does not rule the drain out. Plug the bowl and fill it instead. A cabinet that stays dry while the bowl holds water and goes wet when you pull the stopper has a drain leak, not a supply leak.

It only leaks when water is draining out of the sink.

The trap or one of its slip joints is open. Those are plastic nuts with a tapered washer inside, and they work loose over the years. A plumber re-seats or replaces the trap kit in under an hour.

The leak is coming from the curved U-shaped pipe.

That is the P-trap, and it is the most common leak under any sink. Check the two nuts on each end before you blame the pipe. Most traps leak at a joint, not through the wall of the pipe.

Is a leak under the sink an emergency?

Only when it runs with the water off. That is the supply side leaking under pressure, and it does not stop on its own. Shut both valves under the sink, or the main valve if those will not turn.

How much damage does a slow leak under the sink do?

Far more than the pipe costs. A steady drip swells the cabinet base, lifts the floor under it, and grows mold inside a closed cabinet. Keep the space dry until the repair happens.

Water is leaking under the sink

Published October 5, 2026

Where water shows up matters less than when it shows up. Dry everything under the sink, then test the two systems separately. Water that appears only when the sink drains is the trap or a slip joint, and a plumber swaps those parts in under an hour. Water that is there with the faucet off is on the supply side, it runs all day, and both valves under the sink get shut now.

Tell a supply leak from a drain leak in 10 minutes

Clear the cabinet out. Dry every pipe, every valve and the cabinet floor with a towel.
Lay dry paper towel under the curved trap and under both small shutoff valves at the back wall.
Leave the faucet off for 30 minutes. Check the towels. Anything wet now is a supply leak.
Put the stopper in and fill the sink bowl to the top. Watch the drain pipes for 2 minutes.
Pull the stopper. Watch the whole drain run while the water rushes out.
Run a finger along the underside of each joint and find the highest wet point. Water travels down, so the leak starts above where you see it.

If the towels stay dry until the sink drains, the fault is the trap or a slip joint, and that is a cheap repair. If anything is wet with the faucet off, shut both valves under the sink and call a plumber.

Repair or replace

Situation	What to do
Wet only when the sink drains, at a joint with a visible nut	Repair. A plumber re-seats the tapered washer or fits a new trap kit.
Crack or split through the plastic trap itself	Repair. The trap is a part. The rest of the drain run stays.
Drip at the wide nut where the sink basket meets the drain	Repair. The basket strainer gets a new gasket and fresh plumber's putty.
Wet with the water fully off, at a valve or braided hose	Repair, and today. A supply leak is under pressure every hour of the day.
Shutoff valve weeps at the stem or will not close	Replace the valve. A valve you cannot close leaves you no way to stop the next leak.
Water through the seam of the metal disposal can	Replace the disposal. A leak through the body means the internal seal has gone.
Cabinet floor is soft, swollen or black	Fix the leak first, then rebuild the cabinet base. New pipes over a rotted floor sag and pull the joints open again.

Can I do it myself?

Yes, for a slip joint on the drain side. Shut the water off, put a bucket under the trap, and loosen the plastic nuts by hand. Check each tapered washer for a crack or a bad seat, and reassemble hand-tight plus a quarter turn. Over-tightening a plastic nut splits it, which is the most common way this repair fails.

Hire a plumber when the leak is on the supply side, when the shutoff valve will not close, or when the pipes under the sink are old chrome or metal. Those nuts seize, and one that snaps turns a 20 minute repair into an open wall.

What a contractor will ask you

Have these ready and you skip the back-and-forth.

A wide photo of the whole cabinet, showing the drain pipes, the supply lines and the cabinet floor in one shot.
When you first noticed the water.
Whether it leaks only while the faucet runs, only while the sink drains, or all the time.
Which part is wet: the curved U-shaped trap, the can-shaped disposal, or the small valves and hoses at the back wall.
How old the plumbing under the sink is, or when it was last worked on.
Whether the leaking joint has a plastic nut you can see, or is a glued connection with no nut.
Whether water is still running right now, or you have already shut the valves.

Common questions

It only leaks when the faucet is running.: Running the faucet also sends water down the drain, so this does not rule the drain out. Plug the bowl and fill it instead. A cabinet that stays dry while the bowl holds water and goes wet when you pull the stopper has a drain leak, not a supply leak.
It only leaks when water is draining out of the sink.: The trap or one of its slip joints is open. Those are plastic nuts with a tapered washer inside, and they work loose over the years. A plumber re-seats or replaces the trap kit in under an hour.
The leak is coming from the curved U-shaped pipe.: That is the P-trap, and it is the most common leak under any sink. Check the two nuts on each end before you blame the pipe. Most traps leak at a joint, not through the wall of the pipe.
Is a leak under the sink an emergency?: Only when it runs with the water off. That is the supply side leaking under pressure, and it does not stop on its own. Shut both valves under the sink, or the main valve if those will not turn.
How much damage does a slow leak under the sink do?: Far more than the pipe costs. A steady drip swells the cabinet base, lifts the floor under it, and grows mold inside a closed cabinet. Keep the space dry until the repair happens.