King's Highway 40, commonly referred to as Highway 40, is a provincially maintained highway in the southwestern portion of the Canadian province of Ontario. The 91.4-kilometre (56.8 mi) route links Chatham and Sarnia via Wallaceburg, following close to the St. Clair River. The southern terminus is at Highway 401 south of Chatham, while the northern terminus is at Highway 402 in Sarnia. The portion of Highway 40 between Highway 401 and north of Wallaceburg is within the municipality of Chatham-Kent, while the portion north of there is within Lambton County.

Chatham-Kent is a single-tier municipality in Southwestern Ontario, Canada. It is mostly rural, and its population centres are Chatham, Wallaceburg, Tilbury, Blenheim, Ridgetown, Wheatley and Dresden. The current Municipality of Chatham-Kent was created in 1998 by the amalgamation of Blenheim, Bothwell, Camden, the City of Chatham, the Township of Chatham, Dover, Dresden, Erie Beach, Erieau, Harwich, Highgate, Howard, Orford, Raleigh, Ridgetown, Duart, Thamesville, Tilbury East, Tilbury, Wallaceburg, Wheatley and Zone.