Signal Mountain Blockhouse, Military blockhouse at Signal Mountain, Fort Sill, United States.

Signal Mountain Blockhouse is a small stone building standing at the summit of Signal Mountain at Fort Sill. It was constructed from limestone quarried in the Wichita Mountains and served as a fortified observation post.

Built in 1871 following the Medicine Lodge Treaty, it served the military as an observation post on the frontier. The blockhouse represented a shift in how forces monitored and controlled the Southern Plains region.

The name Signal Mountain comes from the post's signalling role: from 23 June 1875 the station ran atmospheric observations and telegraphic traffic, and the Army Signal Corps used flag semaphore, heliograph and signal lamp before field wire lines arrived. Operators kept line-of-sight contact with Medicine Bluffs, Mountain Scott and Fort Reno, north of the Canadian River.

The blockhouse stands inside the Fort Sill Military Reservation, on the Fort Sill West Range, and access is controlled by the Army; the site is not open to casual visitors. It sits at the 1,750-foot summit of Signal Mountain along Mackenzie Hill Road, north of Lawton in Comanche County. Check current post access rules before planning any approach.

The building was part of an optical communication network using flags, mirrors, and signal lamps to transmit messages. Operators here maintained line-of-sight exchanges with other military posts in the region.