Shower caulk is the perfect mold habitat: it's warm, it's wet most of the day, and it collects a steady diet of soap scum and skin oils. The mistake most people make is treating all caulk mold as a cleaning problem. Whether mold on shower caulk can be removed depends on one thing — whether it's sitting on the bead or living inside it — and there's a two-minute test that tells you which one you've got: clean a small section properly. If the black comes off, keep cleaning. If it doesn't budge, stop scrubbing and skip to the replacement steps below.
Start dry. Wipe the caulk lines down and let them dry. Cleaners cling and work better on a dry surface than on one that's dripping.
Scrub with baking soda paste. Mix baking soda with a few drops of dish soap and just enough water to make a toothpaste consistency. Work it along the caulk with the toothbrush, let it sit 15–20 minutes, then scrub again and rinse. This handles most surface mold and the soap-scum layer feeding it.
For stains that survive the scrub, use a peroxide soak. Soak cotton balls or paper-towel strips in 3% hydrogen peroxide, lay them directly on the stained caulk, and leave them for a few hours or overnight. Contact time does the work here — a spray-and-wipe doesn't. Peel them off and rinse. If the stain is still there after this, it's inside the bead.
Silicone caulk isn't porous like grout, but once its surface degrades and the seal breaks, mold gets behind and inside it, where it's sealed off from every cleaner you own. Black or pink staining that survives the peroxide soak is your verdict. The fix is a full cut-out and re-bead — the same job, with the same technique, as caulking a bathtub like a pro.
Cut out all the old caulk. Slice along both edges of the bead with a utility knife or razor scraper and peel the strip out. Every scrap has to go — new silicone will not stick to old silicone, and leftover film is the number-one reason fresh caulk fails early.
Clean the joint and kill what's left. Scrub the open joint with your baking soda paste, rinse, and wipe it down. This is your one chance to clean the surface the mold was actually living on.
Get the joint bone dry. Silicone won't bond to a damp surface. Wipe the joint with a cloth dampened with rubbing alcohol — it speeds drying and strips soap residue — then point a fan at it and wait until it's completely dry to the touch.
Tape, bead, tool, and peel. Run painter's tape along both sides of the joint, lay a steady bead of 100% silicone kitchen-and-bath caulk, smooth it once with a wet fingertip or a caulk tool, and pull the tape immediately. Then hands off: most silicone needs a full day before it can take a shower, so check the tube and plan the job for a morning when the bathroom can rest.
While you're in there, look at the grout lines honestly. If they're stained and crumbling too, that's a separate job — our guide to how to regrout a shower covers when regrouting beats recaulking, and how to clean grout covers the lines that just need a deep clean.
Mold needs moisture and food, and you control both more than you think. Run the exhaust fan during every shower and for 20–30 minutes after — or open the door and window if there's no fan. A quick squeegee or towel swipe on the caulk lines after the last shower of the day removes most of the water the mold would live on. Fix drippy faucets promptly; a caulk line that never fully dries between showers will mold again no matter how good the caulk is.
Caulking over the old bead. A fresh layer over moldy, failing caulk traps the mold, bonds to a surface that's already letting go, and peels within months. It looks fixed for about two weeks.
Bleaching and calling it done. White caulk isn't clean caulk. If the stain was inside the bead, the mold colony is still there, and the color comes back — often within weeks.
Grabbing latex caulk for the tub. Cheap acrylic/latex caulk shrinks and cracks in wet areas. Kitchen-and-bath silicone is made for exactly this job and carries the mold resistance you want.
Try the paste and the peroxide soak first — surface mold is common and genuinely cleanable. But if the black won't move, believe it: the mold is inside the silicone, and the only fix is cutting the bead out and laying a new one on a clean, dry joint. Do that once, run the fan, and the new caulk should stay clean for years instead of months.
Because the cause is still there: either the mold is inside the old bead (cleaning never reached it), or that spot stays wet — a slow drip, a caulk gap letting water behind the wall finish, or a bathroom that never dries out. Replace the bead and fix the moisture, and the same spot stops being special.
The growth on bathroom caulk is usually common household mold feeding on soap scum, not something exotic — but it's still mold growing where you breathe steam every day, so clean it or cut it out rather than living with it. If anyone in the house has asthma or a weakened immune system, or the moldy area is large and the wall behind it feels soft, that's a job for a professional assessment.
Vinegar handles light surface mold on some surfaces, but it doesn't penetrate silicone any better than bleach does, and it can damage natural stone and some grout. For caulk specifically, the baking soda scrub and peroxide soak work at least as well without the risk. And never layer vinegar over a spot you've treated with bleach.
When it fails, not on a fixed schedule — but in many showers that works out to somewhere around every five years. Replace it when you see cracking, edges pulling away from the tub or tile, or staining that cleaning no longer touches. A bead that still flexes and seals is fine to leave alone.
No. New silicone doesn't adhere to cured silicone, and a layer over a failing bead inherits the failure — plus whatever mold was living in the old caulk stays trapped underneath. Cutting the old bead out is most of the job, and it's the part that makes the new caulk last.