Direct answer: A kitchen sink leaking under the sink is usually coming from one of two places: a pressurized water connection that leaks even when the sink is idle, or a drain connection that only leaks while water is running or the basin is draining. Find the first wet point before you tighten or replace anything.
Most likely: Most often, the leak is at a loose or worn kitchen sink drain connection, basket strainer, P-trap joint, supply line connection, or pull-down faucet hose under the faucet body.
Start with a dry cabinet, a flashlight, and a few short tests. Reality check: the drip you see at the bottom of the cabinet is often not where the leak starts. Common wrong move: blaming the P-trap when the water is actually running down from the faucet hose or supply line above it.
Don’t start with: Do not start by replacing the whole faucet or cranking every nut tighter. Overtightening plastic trap nuts and strainer hardware creates a bigger leak fast.
The cabinet floor gets wet while the sink is sitting idle, or you see slow beads forming on valves, supply lines, or the faucet hose.
Start here: Start with the supply side: dry everything, then inspect the shutoff valves, supply line nuts, and faucet hose connections without running water.
Water appears under the sink when the faucet is on, especially near the faucet body, sprayer hose, or supply lines, but stops when the faucet is off.
Start here: Watch the faucet hose and supply lines while someone runs hot and cold water one at a time.
No leak while filling the sink, but dripping starts once you pull the stopper or let a full basin empty.
Start here: Focus on the basket strainer, sink tailpiece, slip-joint nuts, dishwasher branch tailpiece, and P-trap joints.
A double-bowl sink leaks on one bowl, or the leak shows up when the disposal side drains or when the dishwasher discharges.
Start here: Check the branch tailpiece, disposal discharge connection if present, and the dishwasher drain hose connection before assuming the main trap is bad.
This is the most common drain-side leak. It usually drips only while water is moving through the drain and often leaves a clean wet track below one nut.
Quick check: Dry the trap and tailpiece joints completely, then run water and watch each nut and washer area for the first bead of water.
If water starts high under the sink right below the drain opening, the basket strainer seal may be loose, shifted, or failing.
Quick check: Fill the basin with a few inches of water, then release it and watch directly under the drain flange for seepage before the trap gets wet.
A pressurized leak can show up even when the sink is not being used. Mineral crust, green staining, or a damp valve body are strong clues.
Quick check: With the cabinet dry and the faucet off, wrap a dry paper towel around each supply connection and shutoff valve to find the first damp spot.
Leaks from the faucet base or spray hose often run down the underside of the sink and drip onto the trap, making the drain look guilty.
Quick check: Run the faucet while moving the spray head in and out, then look up under the faucet for water tracking down from above.
You need to know whether the leak is under pressure all the time or only happens when water moves through the sink. That one split cuts the guesswork way down.
Next move: You now know whether to stay on the pressurized side or the drain side, which keeps you from replacing the wrong part. If the whole area gets wet too quickly to tell where it starts, dry it again and test one condition at a time: faucet off, cold only, hot only, then full-basin drain.
What to conclude: A leak with the faucet off points to shutoff valves, supply lines, or the faucet hose. A leak only during draining points to the basket strainer, tailpiece, dishwasher branch, or P-trap connections.
These are common, visible, and easy to confirm without taking the drain apart. They also leak under pressure, so they can damage the cabinet even when the sink is not in use.
Next move: If a slight snug stops the drip and the area stays dry through several hot and cold tests, you likely had a loose connection. If the line keeps weeping, the braided line is kinked, corroded, or damaged, or the valve body is leaking, plan on replacing the failed part rather than tightening harder.
What to conclude: A damp line nut usually points to a kitchen sink supply line issue. A wet valve stem or valve body points to a kitchen sink shutoff valve problem.
A pull-down faucet hose leak is easy to miss because water runs down the underside of the sink and drips onto the trap, making the drain assembly look like the source.
Next move: If you catch water forming on the faucet hose or directly under the faucet body, you have a confirmed faucet-side leak and can ignore the trap for now. If the faucet area stays dry, move down to the drain opening and trap while repeating the test.
Drain leaks should be traced from highest to lowest. If you start at the trap, you can miss water running down from the basket strainer or tailpiece above it.
Next move: If realigning a crooked slip joint stops the leak, run several full drains to confirm it stays dry. If the washer is distorted, the nut is cracked, the tailpiece is split, or the basket strainer seeps from the sink opening, replace the failed drain part.
Once you have the first wet point, the repair is usually straightforward. The key is replacing only the part that actually failed and proving it stays dry under real use.
A good result: If all tests stay dry, the repair is done.
If not: If the same area still leaks after correct installation, the mating part may also be damaged, or the leak may actually be starting higher or farther back than it first appeared.
What to conclude: A successful retest confirms the right repair. If the leak persists or seems to come from the wall, faucet body, disposal connection, or another branch you cannot isolate, it is time for a plumber.
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That usually points to the faucet hose, supply connections under the faucet, or a drain leak that needs moving water to show up. Watch the faucet hose first while the water runs, then move down to the basket strainer and trap.
That is usually a drain-side problem, not a supply leak. The most common spots are the kitchen sink basket strainer, tailpiece, dishwasher branch tailpiece, or a misaligned P-trap slip joint.
Maybe, but do not assume the trap is the source. Water often runs down from the basket strainer or faucet hose and drips off the trap. Find the first wet point first, then tighten or replace only that part.
No. An outside smear is a temporary mess, not a real repair. Slip-joint leaks are usually caused by misalignment, a bad washer, a cracked nut, or a damaged trap piece that needs to be seated correctly or replaced.
Sometimes, if you can shut the water off reliably and the valve is accessible. If the valve is seized, the wall pipe moves, or the valve will not shut off fully, that is a good place to stop and call a plumber.
Slow supply leaks can bead and dry, and faucet-hose leaks can happen only while the faucet is used. Dry everything completely, test one condition at a time, and use a paper towel on each connection to catch small seepage.