State Route 840 is a state highway around Nashville, Tennessee, built and directed by the Tennessee Department of Transportation. First proposed by former Governor Lamar Alexander as part of a system of "Bicentennial Parkways", construction began on this expressway in 1991 – on the first segment extending south from Interstate 40 at Lebanon. The roadway currently serves the cities of Lebanon, Murfreesboro, Franklin, and Dickson. The southern loop, completed in 2012, is about 78 mi (125.5 km) long. The final construction was done on the western half of this semicircle, between U.S. Route 31 near Franklin and Tennessee State Route 100 near Dickson. The Tennessee Department of Transportation completed the final 14 mi (22.5 km) in 2012 and opened it up to traffic on November 2, 2012.

Plans to construct a northern loop of Tenn-840 north of Nashville and past Dickson, Clarksville, Springfield, and Gallatin were put on hold since the Tennessee General Assembly ordered the Department of Transportation to discontinue further studies and planning of that highway because of the current state budget problems and the quite high costs of major highway construction in that area. An entire circular loop would possibly be about 178 mi (286.5 km) long.

Other important objections against additional extensions of the Tenn-840 highway include the hilly nature of the terrain north of Nashville, which would require huge and costly amounts of excavation, soil relocation and bridge construction. In October 2003, the Department of Transportation placed the northern loop project into an indefinite hold situation, citing a lack of documented transportation needs, and lack of participation from local politicians.

While initially referred to as "Interstate 840", or "I-840" by the press, all current official materials refers to the highway as a "State Route". It has been constructed entirely with state transportation funds, and it is not part of the Interstate Highway System. The number 840 was chosen for an easy transition if the road ever becomes Interstate 840 (with Interstate numbers 140, 240, 440, and 640 already being in use in Tennessee).

The route of the southern loop of Tenn-840 passes across five counties. It was announced by TDOT on July 31, 2012 that the unfinished section would be completed and the final 14 mile stretch was opened to traffic on November 2, 2012