Here is the full list of all 37 Psychonauts achievements. It takes around 20-25 hours to unlock all of the achievements on Windows.
Revisit all brains after completion.
Complete Meat Circus.
Achieve Rank 101.
Gather all Figments.
Sort all Emotional Baggage.
Crack all Vaults.
Complete The Asylum.
Introduce all Camp Kids to Mr. Pokeylope.
Re-brain the Children.
See yourself through the eyes of many others.
Complete the Punchy Target mini-game.
Complete Black Velvetopia.
Find all Golden Helmets.
Earn all Merit Badges.
Find Edgar's Secret Garden.
Complete Waterloo World.
Achieve Rank 80.
Buy an item from the Camp Store on Christmas.
Redeem all 16 Scavenger Hunt Items.
Complete Gloria’s Theater.
Cook and consume two different kinds of roast in one sitting.
Complete The Milkman Conspiracy.
Achieve Rank 60.
Complete Lungfishopolis.
Hear Vernon's Ghost Story.
Witness Maloof's transformation.
Spy on Bobby's love life.
Give the Coach's speech on the stump.
Discover the secret history of Whispering Rock.
Read many bulletin board messages.
Complete Milla’s Dance Party.
Achieve Rank 40.
Uncover Milla's Secret.
Complete Sasha’s Shooting Gallery.
Achieve Rank 20.
Use the bacon. A lot.
Complete Basic Braining.