The Lamar River is a tributary of the Yellowstone River, approximately 40 miles (48 km) long, in northwestern Wyoming in the United States. The river is located entirely within the park. It rises in the Absaroka Range, on the eastern edge of the park, and flows northwest through the northeast corner of the park. It is joined by many tributary streams, including Soda Butte Creek and Slough Creek and joins the Yellowstone near Tower Junction, just below the Grand Canyon Of The Yellowstone.
The river is very accessible and meanders through grassy meadows, with riffles, boulders, and undercut banks providing cover for mostly cutthroat trout. More rainbows are found as the river pours through a rocky canyon just before it joins the Yellowstone. The Lamar offers good dry fly fishing with heavy hatches of caddis, pale morning duns, and large Green Drake in July. Terrestrials are prominent in late summer.
All cutthroat trout caught in the Lamar river must be released. Five rainbow trout may be harvested daily.