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