Bombur was one of the twelve companions of Thorin Oakenshield and Bilbo Baggins on the Quest of Erebor. The cousin of Bifur and brother of Bofur, he was not descended from Durin.

"Poor, fat" Bombur is frequently shown as being the last in everything: because he counts for two, he is the last to enter Beorn's house and the last to cross the Enchanted River in Mirkwood. He is also depicted as being foolish and making mistakes when last: he tumbles with Bifur and Bofur onto Thorin when they enter Bag End last, he enters Beorn's house earlier than intended, and he also falls into the Enchanted River. Trusting neither mountain paths nor ropes to hold his weight, he chooses to stay and guard the company's camp while the others move up Erebor. However, he is forced to use the ropes to escape the rampaging dragon Smaug.

Bombur sleeps at several key moments in the book. When he falls into the Enchanted River, he falls under a spell that makes him sleep for days, causing his already despairing companions to carry him. During the siege of Erebor, Bilbo uses Bombur's sleepiness to his advantage, promising to take Bombur's midnight watch and allow him to sleep. As well, he was asleep when his barrel was opened at Esgaroth and when Bilbo discovered the secret entrance to Erebor.

Many years later, in The Lord of the Rings, Frodo Baggins inquires after Bombur and learns that he has grown so fat it takes six young dwarves to lift him, as he can no longer move from his bed to his couch.

In the 1977 Rankin-Bass version of The Hobbit, he was voiced by Paul Frees.