The Absolutely True Diary of a Part-Time Indian has several major themes, but chief among them is the connection between identity and community. The protagonist, Junior, has to redefine himself throughout the book, negotiating between his identity and whatever community he finds himself a part of, be that the Spokane reservation or Reardan High.
No matter where he goes, Junior often feels like a misfit. On the reservation, he is seen as a weakling due to his physical disabilities. He also has more hope than many of the other people there, including his own parents and sister. Junior has dreams for the future, and he goes to Reardan High, an all-white school, to get a better chance of making them come true. However, this decision makes him a pariah among the Spokane Indians, who now see him as a racial traitor. He isn't welcomed with open arms at Reardan either, initially facing prejudice due to his race and poverty. Essentially, Junior is seen as too white on the reservation and as too Indian among white people.
Over the course of the novel, Junior sees his identity as more complicated since he technically belongs to many different "tribes." Identity is not set in stone or defined by one trait, such as race or culture, but by one's choices and friendships. He is a Spokane Indian, a cartoonist, a basketball player, and a great deal more. Realizing how foolish it is to narrow his identity down to just one element, Junior becomes comfortable in his own skin and is able to better navigate life on the reservation and life at Reardan.