Direct answer: An under-sink leak is usually either a drain-side leak that shows up when water is running into the basin, or a supply-side leak that appears even when the sink is not draining. The fastest way to narrow it down is to dry everything, then test one branch at a time and find the first place that gets wet.
Most likely: The most common sources are a loose or worn sink p-trap connection, a leaking sink shutoff valve or supply connection, or a sink strainer leak at the drain opening.
Leaks under a sink often drip from the lowest point, which can make the real source look lower than it is. Start by clearing out the cabinet, drying the area, and separating whether the leak happens under water pressure, during drainage, or both. That usually tells you which component family to inspect first.
Don’t start with: Do not start by replacing multiple parts at once or tightening every nut hard. Over-tightening plastic drain parts and older valves can make the leak worse.
Water appears under the sink while the faucet is on, even before much water reaches the drain.
Start here: Focus on the pressurized side first: supply tube connections, faucet hose connections, and sink shutoff valves.
The cabinet stays dry until you fill the basin or run water long enough for drainage.
Start here: Start at the sink strainer, tailpiece, and sink p-trap joints.
You find a damp cabinet floor or a slow drip even when the sink has not been used recently.
Start here: Look for a seep at a sink shutoff valve body, supply connection, or a hidden faucet hose drip running down from above.
The leak happens only when using one basin, the sprayer, or the dishwasher discharge into the sink drain branch.
Start here: Test that branch by itself so you can isolate whether the problem is the sink strainer, branch tailpiece, sprayer hose, or a nearby drain connection.
This is common when the leak appears only during drainage and the drip forms on plastic slip-joint nuts or the curved trap section.
Quick check: Dry the trap completely, run water for 30 to 60 seconds, and watch each joint with a flashlight for the first bead of water.
If the area gets wet while the faucet is on, or slowly even between uses, the pressurized supply side is a strong suspect.
Quick check: Wipe the valve body, packing area, and supply tube nuts dry, then place a dry paper towel around each point while the faucet runs.
Water can seep around the drain basket seal and run down the underside of the sink, making the p-trap look guilty when it is not.
Quick check: Dry the underside of the drain opening, fill the basin partway, then release the water and watch the underside of the sink around the strainer flange.
A leak from the faucet base, pull-down hose, or side sprayer can drip into the cabinet and mimic a valve or drain leak.
Quick check: With the cabinet dry, run the faucet and sprayer while watching the underside of the faucet and hose loops for fresh drips.
You need to separate a pressure leak from a drain leak before touching fittings. That prevents guesswork and unnecessary part buying.
If it works: You now know whether the leak is pressure-side, drain-side, or from above at the faucet hose area.
If it doesn’t: If water appears everywhere at once or the cabinet is too cramped to see the first wet point, move to a closer visual check with paper towels around one connection at a time.
What that means: A leak during faucet use points to supply valves, supply tubes, or faucet hoses. A leak during drainage points to the sink strainer, tailpiece, or sink p-trap.
Supply-side leaks can damage cabinets even when small because they may seep under pressure between uses or whenever the faucet is turned on.
If it works: If the leak stops after a slight connection snugging and stays dry through repeated tests, the issue was likely a loose connection.
If it doesn’t: If the valve body, stem, or wall-side connection still seeps, the valve branch is likely worn or disturbed and may need replacement.
What that means: A wet valve body or stem usually points to a failing sink shutoff valve, while a wet outlet nut may point to a connection issue or supply tube seating problem.
Drain leaks are usually easier to isolate because they happen only when water is moving through the drain assembly.
If it works: If a slightly loose slip-joint connection stops leaking and stays dry through a full-basin drain test, the repair may be complete.
If it doesn’t: If the leak continues from a cracked trap, distorted washer joint, or the sink strainer area, that branch likely needs a targeted replacement or reseal.
What that means: Water at the drain opening points to the sink strainer. Water at a trap joint points to a washer, alignment, or sink p-trap issue. A crack means the affected drain piece should be replaced.
A drip from the faucet underside can travel down hoses and make lower fittings look like the source.
If it works: If you find a hose leak above the valves and drain, you have isolated the branch and can ignore lower drips caused by runoff.
If it doesn’t: If no hose or faucet underside leak appears, return to the pressure-side or drain-side branch that matched your earlier test.
What that means: A leaking sink sprayer hose or faucet hose often causes intermittent cabinet water and is easy to confuse with a valve or trap leak.
Once you know the exact branch, you can choose one specific part instead of replacing several things blindly.
If it works: You avoid buying the wrong part and can verify the fix with a clean retest.
If it doesn’t: If the source is still uncertain after isolated testing, the safest next step is a plumber, especially if the leak may be inside the wall or at an older valve.
What that means: A confirmed first wet point is the best predictor of the correct repair. The final drip location is often misleading.
Only use these links after your checks point to the part that actually failed.
Buy only if the sink p-trap or one of its trap sections is cracked, warped, or still leaks after confirming the joint and washer alignment are correct.
Buy only if water is clearly seeping from the underside of the sink drain opening or the strainer body is damaged or corroded.
Buy only if the sink shutoff valve body or handle stem is leaking, or the valve will not shut off reliably and you have confirmed an upstream shutoff is available for replacement.
Buy only if the leak appears only when using the sprayer or you find a visible split or seep in the sink sprayer hose or its connection.
That usually points to the drain side, not the pressurized supply side. Common sources are the sink strainer, tailpiece, or sink p-trap joints. Dry those parts first and watch for the first place that gets wet during a drain test.
A slow seep at a sink shutoff valve, supply connection, or faucet hose can leave water in the cabinet between uses. It can also be runoff from a small leak above that drips down later. Dry everything and check again before and during faucet use.
Sometimes, but only lightly and only after you know which connection is actually leaking. Over-tightening plastic drain nuts can crack fittings, and forcing older supply fittings or valves can make the leak worse.
Dry the underside of the sink around the drain opening and the trap below it. Then fill the basin and let it drain while watching the underside of the drain opening first. If that area gets wet before the trap, the sink strainer branch is the better match.
Only if you can shut water off upstream, the valve is accessible, and the connection style is clear to you. If the valve is seized, corroded, twisting at the wall, or the leak may involve the wall-side connection, it is safer to call a plumber.