Katmai is representative of the northern portion of the Alaska Peninsula and contains portions of two physiographic provinces: the Aleutian Range and the Nushagak-Bristol Bay Lowlands. The Bruin Bay Fault, one of the major faults of Alaska, separates the two geologically different portions.

The Aleutian Range province is characterized by three landforms: the Shelikof Strait Seacoast, the Aleutian Range and the lake region centered around Naknek Lake. The Shelikof Strait seacoast is a rugged, diversified area of narrow-to-wide bays, long and narrow-to-wide beaches, and intricate covers. Steep cliffs rising from the bays are common along the coastline. Rivers cascade down steep canyons, and waterfalls plunge onto ocean beaches. Deep blue water, pale bluff pumice and the green of alder patches and grasslands are typical of the coast.

The Aleutian Range is the backbone of the Alaska Peninsula. The higher peaks of this range within the Katmai National Park were formed predominately by volcanic action and rise steeply from the Shelikof Strait coastline to altitudes greater than 7,000 feet. Mount Dennison, 7,606 feet, is the highest elevation in Katmai. The slopes and upper valleys surrounding these peaks contain glaciers on both sides of the Aleutian divide. A few of these glaciers descend on the east almost to sea level. The largest glaciers in the park are three to four miles wide and 10 to 12 miles long. The only travel routes of low relief across this portion of the Aleutian Range are at Becharof Lake, at Katmai Pass, at Kaguyak Pass, and through the lake country in the northern part of the park.

The north-central and northwestern portion of the Katmai are commonly termed "the lake region". Naknek Lake is the principal part of a hydrologic system of lakes, ponds, rivers, streams, and marshes formed in valleys dammed by glacial deposits. Lakes in the eastern portion of this region are bordered by mountains that rise to 3,000 feet above the water. The western part of this area is open terrain and grades into the Bristol Bay coastal plain.

The southwestern portion of the unit is part of the Bristol Bay coastal plain, the only portion of the Nushagak-Bristol Bay Lowlands physiographic province represented in the unit. The terrain is relatively flat, with many poorly drained lakes. A number of low ridges, sand dunes, and streams break the uniformity of this lowland expanse. At least part of the coastal plain and scattered parts of the lake region are underlain by permafrost, which is ground that has a temperature continuously below 32 degrees Fahrenheit for more than two years.