US-377 crosses Lake Texoma on a bridge from Grayson County, Texas into Marshall County, Oklahoma. This is the southern terminus of SH-99, which will concur with US-377 all the way to the U.S. highway's northern terminus in Stroud, a distance of . The highways' first junction in Oklahoma is with State Highway 32 south of Madill. Five miles north of this intersection, US-377/SH-99 serve as the northern terminus for SH-99C, a child route of SH-99. The routes then head into Madill, where they form a brief concurrency with US-70 and SH-199. US-377/SH-99 head northeast out of town and enter Johnston County.