In Minecraft, the chance of getting a trident depends on the edition and whether the Drowned is actually holding one. In Java Edition, a Drowned has about a 6.25% to 6.5% chance to spawn holding a trident, and if it is holding one, the drop chance is about 8.5% before Looting bonuses.

Drop chance by edition

EditionDrowned spawns holding tridentTrident drop chance
Java6.25% to 6.5%8.5% base, up to 11.5% with Looting III
Bedrock15%25% base, up to 37% with Looting III

What that means

You cannot get a trident from just any Drowned; it must be the right kind of Drowned, and in Java that is much rarer. Bedrock is noticeably easier because more Drowned can spawn holding tridents and the drop rate is higher.

Best way to farm it

  1. Hunt Drowned near rivers, oceans, and ocean ruins.
  2. Use a Looting-enchanted weapon to improve the odds.
  3. Focus on Drowned that are visibly holding tridents, since those are the ones that can drop one in Java.

If you want, I can also give you the easiest trident-farming method for your edition.