A sink that smells like sewer gas may have a dry trap, organic buildup near the drain, poor venting, or a piping problem. Separating odor from the sink itself from gas coming through the drain prevents repeated cleaning that misses the cause.
Run water if the sink is rarely used
A trap needs water to block sewer gas. If the sink has been unused for weeks, run water and see whether the odor disappears after the water seal is restored.
Clean the overflow and stopper area
Biofilm, soap, and trapped debris can create an odor that is mistaken for sewer gas. Clean accessible surfaces without mixing chemicals, then reassess before taking plumbing apart.
Watch for gurgling or slow drainage
A sink that gurgles, drains slowly, or changes behavior when another fixture runs may have a partial blockage or venting issue. Those clues matter more than odor alone.
A leak under the sink can smell similar
Persistent moisture inside the cabinet can create moldy or organic odors. Check the trap, connections, and cabinet floor; if water or damage is present, fix the leak instead of treating the smell alone.