stamp collector; philatelist
A person who collects postage stamps as a hobby or for study. 切手収集家 is the formal term; in casual conversation, 切手コレクター is also common.
He is an enthusiastic stamp collector and owns rare stamps from around the world.
My grandfather was known as a stamp collector since he was young.
More casual, loanword-based synonym for 'stamp collector'. 切手収集家 is more formal and often used in written contexts.
Compound of 切手 (kitte, 'stamp') + 収集 (shūshū, 'collection') + 家 (ka, 'person who does something as a profession or hobby').