In geology, a boulder is a rock with grain size of usually no less than 256 mm (10 inches) diameter. While a boulder may be small enough to move or roll manually, others are extremely massive. In common usage, a boulder is too large for a person to move. Smaller boulders are usually just called rocks or stones. The word boulder comes from Middle English "bulder" which was probably of Scandinavian origin such as dialectal Swedish "bullersten" meaning "noisy stone" (Imagine a large stone in a stream, causing water to roar around it) from "bullra" (to roar, cf. Dutch "bulderen", with the same meaning) and "sten" (stone).

In places covered by ice sheets during Ice Ages, such as Scandinavia, northern North America, and Russia, glacial erratics are common. Erratics are boulders picked up by the ice sheet during its advance, and deposited during its retreat. They are called "erratic" because they typically are of a different rock type than the bedrock on which they are deposited. One of the largest is used as the pedestal of the Bronze Horseman in Saint Petersburg, Russia.

Some noted rock formations involve giant boulders exposed by erosion, such as the Devil's Marbles in Australia's Northern Territory, the Wairere Boulders in New Zealand, where an entire valley contains only boulders, and The Baths on the island of Virgin Gorda in the British Virgin Islands.

The climbing of large boulders often requires months or even years of practice, and has given rise, since the late 19th century, to the sport of bouldering.