Quick answer
A musty smell under a bathroom sink means something under there has stayed damp long enough to grow mold or mildew: a slow drip from a supply line or trap, a leaking drain seal, or water reaching the cabinet from a pipe in the wall. Empty the cabinet and find where the smell is strongest, run leak tests on the supply and drain, and look under the cabinet floor and at the bottom of the wall behind it. A smell from the drain opening is usually biofilm in the overflow or a dry trap. Swollen particleboard should be replaced, and mold on the subfloor or wall needs removal after the leak is fixed. Call (888) 840-6512 to reach a certified water damage restoration company in your area; the line is answered 24/7.
Press 1 for water damage or 2 for mold, then your 5-digit ZIP. The line goes quiet for about half a minute while it connects — that is normal, stay on. If it does not connect, call once more: companies taking calls change minute to minute.
Written & reviewed by Marcus ReedIICRC WRT
Reviewed October 20, 2026· Next review Apr 2027
A musty smell under a bathroom sink means something under there has been damp long enough for mold or mildew to grow: a slow drip from a supply line or trap, a leak from the drain seal, or water getting into the cabinet base from somewhere else. If you cannot find a leak, the smell itself is the evidence that one is there or was, and the place to look is under the cabinet floor and inside the wall behind it.
Find the Source
- 1Empty the cabinet and smell close up. A smell strongest at the cabinet floor points to the particleboard base; strongest at the back wall points to the wall cavity; strongest at the drain opening points to the drain.
- 2Run the leak tests. Dry every fitting, put paper towel on the cabinet floor and leave the faucet off for ten minutes, then fill and drain the basin. Our leak under bathroom sink guide has the full test.
- 3Look under and behind the cabinet floor. Many vanities have a loose base panel or an access hole around the pipes. Use a flashlight and look for dark staining, swelling or visible growth on the subfloor and the bottom of the drywall.
- 4Rule out the drain. A smell that comes from the drain opening, not the cabinet, is usually a dry P-trap, a biofilm-coated overflow or the pop-up assembly. Run water and clean the overflow; that smell is a drain problem, not mold.
What the Smell Usually Means
| Where the smell is | What it usually means | What to do |
|---|---|---|
| At the drain opening | Biofilm in the overflow or pop-up; dry trap | Clean the overflow and drain; run water |
| Inside the cabinet, no leak found | An old leak that soaked the cabinet base | Replace the base; check the subfloor |
| At the back wall | A leak in the wall: supply pipe, shower valve, toilet line | Plumber to trace it |
| Same smell in the room below | Water has reached the floor or ceiling below | Moisture readings and drying |
| Sewage smell, not musty | Sewer gas: dry trap, failed vent, cracked drain | See bathroom smells like sewer |
Talk to a water damage pro now
Answered 24/7 · Toll-free · no obligation
(888) 840-6512- Press 1 for water damage or 2 for mold, then your ZIP
- Stay on through the short quiet while it connects
- A local certified company explains the job and price
When to Call for Help
Replace a swollen or stained cabinet base rather than cleaning it; particleboard does not dry back. If you find mold on the subfloor or the drywall behind the cabinet, or the area is larger than a cabinet's footprint, that is a mold remediation and drying job. The leak gets fixed by a plumber; the mold and the wet materials are handled by a water damage or mold company.
(888) 840-6512 is answered 24/7 and connects you to an IICRC-certified water damage restoration company taking calls in your ZIP; there is no charge for the referral. Press 1 for water damage or 2 for mold, enter your 5-digit ZIP, and stay on through about half a minute of quiet while it connects. If it does not connect, call once more: the companies taking calls change minute to minute.
FAQ
Why does under my bathroom sink smell musty but there is no leak? Either the leak was there and is now dry, leaving mold in the cabinet base, or it only leaks during use. Check the overflow and drain too, because biofilm there smells similar.
Is a musty smell under the sink dangerous? It means mold is likely growing, which can irritate allergies and asthma. Find and remove the source.
How do I get rid of a musty smell under the bathroom sink? Fix any leak, replace swollen particleboard, clean or remove any mold, and let the cabinet dry with the doors open. Air fresheners only hide it.
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Frequently Asked Questions
Why does under my bathroom sink smell musty but there is no leak?
Either an old leak soaked the cabinet base and left mold, or it only leaks during use. Check the overflow and drain too, because biofilm there smells similar.
Is a musty smell under the sink dangerous?
It means mold is likely growing, which can irritate allergies and asthma. Find and remove the source rather than masking it.
How do I get rid of a musty smell under the bathroom sink?
Fix any leak, replace swollen particleboard, clean or remove mold, and let the cabinet dry with the doors open. Air fresheners only hide it.
What is the difference between a musty smell and a sewage smell under the sink?
Musty and earthy points to mold from damp. A rotten-egg or sewage smell points to sewer gas from a dry trap, failed vent or cracked drain.
Who do I call right now for water damage?
Call (888) 840-6512. The line is answered 24 hours a day, seven days a week, and connects you with an IICRC-certified water damage restoration company that serves your area. A recorded menu picks up first: choose water damage or mold, key in your 5-digit ZIP code, and hold through the short silence while the call is passed to the company.
Describe what you can see — where the water is coming from, how long it has been there, and whether it smells — the more specific you are, the faster the right company is on the line. If the source is a pipe or an appliance, shut the main water valve first. Have four things ready and the call goes quickly: the address, when the water started, what is wet now, and whether you have filed an insurance claim.
Still not sure which of these is your situation? (888) 840-6512 · toll-free · answered 24/7
Sources
- EPA — A Brief Guide to Mold, Moisture and Your Home
- ANSI/IICRC S520 Standard for Professional Mold Remediation
Methodology: How we source and verify data · Report an error
Disclaimer: HearthDry is an independent educational resource. This article is for informational purposes only and does not constitute professional, legal, or insurance advice. Consult licensed professionals before making decisions about your property or insurance claims.
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