Direct answer: If it smells damp under the sink, assume moisture is still present until you prove otherwise. Most of the time the smell comes from a slow supply leak, a drain connection seep, wet cabinet material, or condensation collecting where air does not move.
Most likely: The most likely cause is a small plumbing leak that only wets the cabinet a little at a time, especially around the sink drain, P-trap joints, shutoff valves, or faucet supply connections.
Start by emptying the cabinet and figuring out whether you have an active leak, leftover damp material, or a drain-related smell with no visible water. Reality check: a cabinet can smell musty long before you ever see a puddle. Common wrong move: caulking around plumbing holes or laying down shelf paper before the wet source is fixed.
Don’t start with: Don’t start with odor sprays, bleach, or fresh liner paper. If the wood or cabinet floor is still getting wet, the smell will come right back.
The cabinet smells stale or musty all the time, and the cabinet floor, side walls, or stored items may feel cool or slightly damp.
Start here: Start with a dry-paper test on the cabinet floor, side panels, and around shutoff valves and supply lines to look for a slow leak or old moisture trapped in the wood.
The odor spikes after washing hands, doing dishes, or draining a full sink, even if you do not see standing water.
Start here: Start at the sink drain, tailpiece, P-trap joints, and any slip nuts. Run water while watching and feeling for seepage.
Pipes may sweat, the cabinet feels warmer or muggy, and the smell is more noticeable in warm weather or after hot water use.
Start here: Check for condensation on cold water lines, the trap, and the underside of the sink, especially where airflow is poor.
The plumbing may now be dry, but the cabinet base, particleboard, or trim still smells sour or musty.
Start here: Check whether the cabinet material is still swollen, soft, darkened, or damp below the surface. Old wet cabinet material can hold odor long after the drip stops.
This is the classic under-sink smell source because it wets the cabinet in tiny amounts that evaporate slowly and never make a dramatic puddle.
Quick check: Dry every drain fitting, then run warm water for several minutes and wipe each joint with toilet paper or a dry paper towel to catch a fine seep.
A tiny pressure-side leak can leave the cabinet floor damp, stain the wall or floor of the cabinet, and create a steady musty smell.
Quick check: Feel around the faucet supply connections, shutoff valve stems, and compression nuts for moisture, mineral crust, or greenish staining.
In humid rooms, cold water lines and metal traps can sweat enough to keep the cabinet damp without any plumbing failure.
Quick check: Look for uniform beads of water on cold surfaces rather than one wet spot below a fitting or joint.
Particleboard, plywood edges, and cabinet toe-kick areas hold odor after they have been soaked, especially where air does not circulate.
Quick check: Press gently on the cabinet floor and side panels. Swelling, softness, dark staining, or a sour smell in one spot points to retained moisture in the material.
You need a clean view before you can tell whether the smell is from a live leak, condensation, or cabinet material that never dried out.
Next move: If you immediately find a wet area, you have narrowed the source zone and can trace it instead of guessing. If everything looks dry, keep going. Many under-sink odor problems only show up while water is running or after the cabinet sits closed.
What to conclude: A damp smell with no obvious puddle usually means a very small leak, sweating pipes, or cabinet material that stayed wet after an earlier event.
Supply leaks are easy to miss because they can be tiny, steady, and hidden behind the sink bowl or against the back wall of the cabinet.
Next move: If a paper towel under a valve or supply line gets wet, you have a live supply-side leak that needs repair before cleanup matters. If the supply side stays dry, move to the drain side. Drain leaks are the next most common cause of a damp cabinet smell.
What to conclude: Wetness on the pressure side points to a shutoff valve, supply line, or faucet connection seep rather than a trap odor or old cabinet smell.
Drain leaks often only show up when the sink is actually draining, and they can leave just enough moisture to create odor without a visible puddle.
Next move: If a joint or drain seal starts weeping during use, you have found the source of the damp smell and can repair that leak first. If the drain side stays dry, the smell is more likely from condensation, a hidden leak above, or cabinet material that stayed wet from an older problem.
Not every damp smell is a plumbing failure. In humid rooms, cold pipes and poor airflow can keep the cabinet damp enough to smell musty.
Next move: If the smell fades as the cabinet dries and no fresh leak appears, trapped moisture or condensation is the likely cause. If odor returns quickly with no visible sweating, look again for a hidden leak above, around the faucet base, sink rim, or behind the wall side of the cabinet.
Odor cleanup only lasts if the moisture source is stopped and the cabinet is dried all the way through.
A good result: If the smell stays gone for several days of normal use, you solved the source problem and the cabinet has dried enough to recover.
If not: If the smell returns after confirmed drying and no new leak is visible, the odor is likely trapped in damaged cabinet material or there is still a hidden leak that needs a closer inspection.
What to conclude: A cabinet that stays smelly after the leak is fixed usually has absorbed moisture damage, not just a surface odor problem.
Usually because the leak is tiny, intermittent, or already soaked into the cabinet material. Drain joints may only seep while water is running, and supply fittings can leave just enough moisture to smell without making a puddle.
Yes, but that is usually more of a sewer or stale drain smell than a true damp or musty smell. If the odor gets stronger when water drains, check both for a small trap leak and for drain-related odor near the sink opening.
Not until you know the cabinet is staying dry. New liner paper over damp wood or particleboard traps moisture and often makes the smell come back faster.
The cabinet material may have absorbed water and odor. Dry it thoroughly first. If it stays swollen, soft, or sour-smelling after drying, the damaged panel or cabinet base usually needs repair or replacement.
Yes. In humid conditions, cold water lines, metal traps, and the underside of the sink can sweat enough to keep the cabinet damp. Condensation usually shows up as even moisture on cold surfaces rather than one wet point below a fitting.
Call a plumber if the leak is inside the wall, at a seized or corroded valve, or from a fitting you cannot access safely. Call a restoration or repair pro if cabinet damage is extensive, the wall or floor is involved, or mold growth is more than a small localized area.