A leak under a sink can come from the trap, a slip-joint connection, the sink drain, or a pressurized supply line. Before tightening anything, dry the area and identify exactly where the first drop appears.
Separate drain leaks from supply-line leaks
A leak that appears only while water drains is usually on the waste side; one that drips even when the sink is unused may come from the supply. If it is a pressurized line, close the appropriate shutoff valve before working on it.
Clean and reseat trap slip-joint washers
On removable traps, a crooked washer or debris on the sealing surface can cause a drip. Work over a container, clean the parts, orient the washer correctly, and hand-tighten with only a gentle final adjustment if the material allows it.
Do not overtighten plastic fittings
Plastic nuts can crack or distort their washers when forced with tools. If a connection still leaks after being reseated, replacing a damaged washer or fitting is usually better than tightening harder.
Escalate if there is corrosion, pressure, or concealed damage
Corroded piping, shutoff valves that will not close, water coming from the wall, or swollen cabinetry points to a larger problem. Keep the supply off when needed and call a plumber if you cannot isolate the leak safely.