The Grand Prismatic Spring in Yellowstone National Park is the largest hot spring in the United States, and the third largest in the world, after Frying Pan Lake in New Zealand and Boiling Lake in Dominica.

The "biggest" thing in Yellowstone National Park depends on whether you are looking at geology, biology, or the park's sheer scale. Geologically, the biggest feature is the Yellowstone Caldera, an active supervolcano measuring approximately 34 by 45 miles (55 by 72 km). It is so massive that it can only be fully appreciated from space; most visitors are standing right in the middle of it without realizing it. Biologically, the biggest animal is the Bison (American Buffalo), with males weighing up to 2,000 pounds, making them the largest land mammals in North America. In terms of water features, Yellowstone Lake is the largest high-elevation lake in North America, covering 132 square miles with over 140 miles of shoreline. If you consider "living" structures, the Old Faithful Inn is often cited as the largest log structure in the world, a masterpiece of "National Park Service Rustic" architecture. Finally, the park itself is a giant, spanning 3,472 square miles—larger than the states of Rhode Island and Delaware combined—making the entire ecosystem the "biggest" entity of its kind in the lower 48 states.