How to Get Sweat Stains Out of Hats?

Step by step (fabric baseball / dad cap)

1. Mix cool water with a little dish soap or a sports-gear / enzyme cleaner (sweat is salt + oil + protein).
2. Dip a soft brush or cloth — not a flood of water into the crown.
3. Scrub the sweatband and the stained brim underside. Lift dirt; don't grind it into foam panels.
4. Wipe with a clean damp cloth until the soap is gone. Leftover soap attracts more dirt.
5. Air-dry on a can, ball, or towel stuffed in the crown. Heat in a dryer shrinks and bends bills.

White crust (salt): a 1:1 white vinegar and water wipe on the band, then soap, can help after you blot.

Wool / felt / western hats: skip soaking. A hat brush, a tiny bit of wool cleaner, or a professional cleaner. Water rings are hard to reverse at home.

Straw: wipe; don't soak. Structured plastic-buckram bills hate dishwashers even if TikTok says otherwise — glue and cardboard fail.

Prevention: a cheap sweat liner in the band, and let the hat dry between wears.

The closest sibling pages are How to Polish Stainless Steel? and How to Remove Dog Hair From Clothes?.

See other Cleaning & Stain Removal questions for other answers in this topic, or start from more researched answers.