Interstate 90 enters Wyoming from Montana and heads south concurrent with US 87 through hilly grasslands. It curves to the southeast and meets an interchange that serves the small community of Parkman. Continuing southeast, the highway intersects US 14 at a diamond interchange. US 14 joins I-90 and US 87 and the three routes curve east. After the freeway bends back to the southeast, it intersects the eastern end of WYO 345, then crosses the BNSF Railway and the Tongue River. The highway serves as the western terminus of WYO 339 and then passes over WYO 338 without an intersection just before it crosses Goose Creek. Just south of Goose Creek, US 14 and US 87 split off to the south of I-90 via a trumpet interchange. This road is also signed as a business loop of I-90. The freeway turns due south and skirts around the eastern edge of Sheridan. It intersects WYO 336, then crosses over the BNSF Railway again, before intersecting US 14 farther south. South of Sheridan, I-90 continues to travel south roughly parallel to US 87. The highway intersects WYO 342 and Prairie Dog Creek Road at diamond interchanges before US 87 merges back into and becomes concurrent with I-90. The highway runs southwest of Lake Desmet. Here it meets Lake Ridge Road, which leads to the lake. Southeast of Lake Desmet, I-90/US 87 comes to a diamond interchange with Rock Creek Road. It then intersects the business loops of US 87 and I-25 at an incomplete diamond interchange; only traffic heading southbound can exit, and traffic from the business loops can only access I-90 northbound. Just southeast of this interchange, I-90 meets the northern terminus of I-25. US 87 follows I-25 southbound toward Buffalo, while I-90 bends to the southeast and intersects US 16 at a diamond interchange. Southeast of Buffalo, I-90 continues in a general southeasterly direction through hilly grasslands. The highway intersects several minor farm roads, then curves to the northeast toward Gillette. Just west of the city, the freeway turns due east. Within Gillette, I-90 comes to a diamond interchange with WYO 50 and its own business loop, then dips to the southeast and meets another diamond interchange, this one with WYO 59. I-90 then has one last exit in Gillette: a partial cloverleaf interchange with US 14, US 16, and WYO 51. The first two of those routes become concurrent with I-90 and follow it east. The freeway crosses the BNSF Railway and then turns to the east. It meets a diamond interchange with Garner Lake Road, then leaves Gillette and intersects American Road before crossing the BNSF Railway again. The highway travels east, parallel to WYO 51 and then meets Adon Road, then curves to the southeast toward Moorcroft. I-90 has two exits in Moorcroft; at the first exit, US 14 and US 16 leave I-90, and I-90's business loop begins. The highway passes over US 14 with no access and then meets its second exit in the city: the eastern terminus of its business loop. East of this exit, I-90 leaves the city. East of Moorcroft, I-90 intersects several more minor rural roads while it bends to the northeast toward Sundance. As it nears Sundance, it shares a diamond interchange with US 14 and BL 90. The route then curves to the east and enters the city, intersecting WYO 585 at a diamond interchange. I-90 then meets US 14 and the eastern terminus of BL 90; the former becomes concurrent with I-90, and the two routes head northeast, leaving Sundance. Northeast of Sundance, I-90/US 14 parallels the former alignment of the latter. The highway intersects Sundance-Moskee Road, then bends north and meets WYO 111. I-90/US 14 then has one last exit, with Sand Creek Road, before crossing into South Dakota.
Exit list
Business routes
s are present in Sheridan, Buffalo, Gillette, and Sundance.
