If you're a USA Citizen then yes you'll need a US Passport to enter and leave Tahiti.
The USA Passport must be Valid for 6 months before you plant to arrive in Tahiti and when you plan to depart Tahiti.
So make sure you get your passport at least 6 months before entering Tahiti and make sure the Passport stays valid so you can leave Tahiti as well and reenter the USA.
Tahiti is a place I want to visit before I die and I hope you get to go to Tahiti as I hear it's a wonderful place to visit.