Direct answer: A bathroom sink supply line usually leaks at one of three spots: the top faucet connection, the bottom shutoff connection, or from a split or corroded section in the line itself. Dry everything first, then run the faucet and trace the first wet point instead of chasing the drip on the cabinet floor.
Most likely: Most of the time, the leak is a loose compression nut or an aging braided bathroom sink supply line that has started seeping near one end.
Under-sink leaks like to fool you. Water runs down the line, along the shutoff, and off the bottom of the cabinet, so the puddle is rarely the real source. Reality check: a tiny steady seep can soak a vanity fast. Common wrong move: tightening every nut hard enough to twist the valve or crack an older connection.
Don’t start with: Do not start by replacing the whole faucet or smearing sealant on the outside of the line. That wastes time and usually does not stop a pressure-side leak.
Water beads around the nut where the bathroom sink supply line meets the shutoff valve, then drips to the cabinet floor.
Start here: Dry the valve body, the nut, and the line completely, then watch for fresh water forming right at the lower connection.
The line looks wet near the faucet shank or faucet hose connection under the sink.
Start here: Run a dry paper towel around the top connection first to see whether the leak starts there and runs downward.
The braided bathroom sink supply line is damp or has green, white, or rusty staining along the hose itself.
Start here: Check for a split jacket, corrosion at the crimp, or a pinhole seep in the line body. That line is replacement territory, not a tightening job.
You see water under the vanity after handwashing, but not when the sink sits unused.
Start here: Dry the supply lines and then run water while watching the drain flange, pop-up rod area, and trap so you do not confuse a drain leak with a supply leak.
This is the most common pressure-side leak under a bathroom sink, especially after someone bumped the line or recently worked under the vanity.
Quick check: Dry the lower nut and valve, then watch for a fresh bead forming exactly at the nut-to-valve connection.
If the top of the line gets wet first, water will run down the hose and make the whole assembly look bad.
Quick check: Wrap a dry tissue around the top connection and run the faucet for a minute. If the tissue wets there first, the upper connection is the source.
Older braided lines can seep at the crimped ends or split internally and show dampness along the hose body.
Quick check: Look for corrosion, bulging, frayed braid, mineral crust, or a wet spot in the middle of the line after everything else is dry.
A drain flange or pop-up leak often shows up only while the faucet is running, which gets mistaken for a supply problem all the time.
Quick check: With the supply lines dry, run water and watch the drain flange, tailpiece, and pop-up rod opening for the first drip.
You need the real source before touching any nut. Under-sink leaks travel.
Next move: You spot the first place water forms, which tells you whether this is really a supply-line leak or a drain-side leak. If everything stays dry but the cabinet floor keeps getting wet later, check for splashing at the sink rim or water running down from the countertop or backsplash.
What to conclude: A leak with the faucet off points to a pressurized connection or shutoff valve. A leak only during use may still be supply-side, but drain leaks are common lookalikes.
A bathroom sink drain flange leak can mimic a supply-line problem and send you after the wrong part.
Next move: You rule in the bathroom sink supply line or rule it out before buying anything. If both areas get wet at once, start with the highest wet point under pressure, because water from above can make the drain look guilty.
What to conclude: Pressure-side leaks usually show up at the line or valve even when the sink is not draining. Drain leaks usually wait until water goes down the bowl.
A slightly loose connection is common, and a small correction may stop the seep without replacing parts.
Next move: If the seep stops and the connection stays dry, you likely had a loose nut and can move to verification. If the nut is already snug, the leak returns right away, or the line body itself is wet, stop tightening and plan on replacing the failed part.
If the hose is seeping, corroded, kinked, or leaking at a crimp, replacement is the durable fix.
Next move: If both ends stay dry and the hose body stays dry under pressure, the supply line was the failed part. If the new line still leaks at the valve connection or the valve stem starts seeping, the shutoff valve itself is likely the problem.
When water forms on the valve body or around the outlet even with a good supply line, the valve is no longer sealing reliably.
A good result: A dry valve and dry supply-line connection after replacement confirm the shutoff valve was the source.
If not: If water still appears from above after the valve is dry, go back to the faucet-side connection. If the leak is in the wall, bring in a plumber.
What to conclude: A leaking valve will keep fooling you until it is fixed. Once the valve is dry, any remaining moisture usually points to the upper connection or another nearby leak.
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Sometimes, yes, if the leak is from a slightly loose connection nut. Use a small snug only. If the line body is wet, the crimp is leaking, or the nut is already snug, replace the bathroom sink supply line instead of forcing it tighter.
A small pressure-side seep may show up more after the faucet is used because pressure changes and vibration make it easier to spot. A leak that appears only during drainage may not be the supply line at all and is often the drain flange or pop-up area.
Dry everything and watch for the first wet point. If water forms on the valve body or around the valve stem, the bathroom sink shutoff valve is the problem. If the hose or its end fitting gets wet first, the supply line is more likely.
If both lines are the same age and material, replacing both is often smart while you are already under the sink. But diagnosis still comes first. Do not assume the second line is bad unless it shows age, corrosion, or damage.
It can become one fast. Even a slow drip can damage the vanity and floor, and a weak braided line can fail suddenly. If you cannot stop the leak cleanly at the shutoff valve, shut off the house water and get help.