The smell comes from bacteria, and a shower doesn't remove all of them.

Underarm odor is made by bacteria feeding on secretions from your armpit glands, and those secretions have almost no smell when they come out (source). Researchers showed this decades ago by putting underarm bacteria on fresh secretions, and the familiar smell appeared (source).

A shower washes away the bacteria sitting on the surface of your skin. Not all of them live there, though. Some live deeper, in hair roots and pores (source). Studies of surgical skin cleaning found that bacteria survive in those deeper spots even after strong antiseptics, and that they can spread back to the surface afterwards (source, source). Those studies looked at skin prepared for surgery, not at armpits after a shower, but soap and water is gentler than an antiseptic, so a shower likely doesn't reach them either.

Your armpit also tends to host a stable mix of the same kinds of bacteria (source). And your glands keep releasing new secretions through the day, for example when you're stressed or excited (source). So the bacteria get a fresh supply of food long after your shower.

What helps is simple. Dry your underarms completely before you put anything on, since products are meant for clean, dry skin (source). On long days, washing again and reapplying at midday is a sensible routine. That last part is practical advice that follows from the science above, not something a study has measured.

You're clean. Your armpits just have company.

Some of these studies are small, and some looked at skin other than the armpit. We say so where it matters.

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