SR 20 begins in Dyer County in Dyersburg as the unsigned companion route go US 412 at an expansive parclo interchange with US 51/US 412/SR 3/SR 211. US 412/SR 20 go south as a 4-lane freeway to a diamond interchange with SR 104, where it downgrades to a 4-lane expressway just before crossing the North Fork of the Forked Deer River and leaving Dyersburg. They then curve to the southeast as they pass through farmland and have a diamond interchange with SR 210. US 412/SR 20 continue southeast and cross into Crockett County.
Crockett County
US 412/SR 20 continue through farmland to the town of Friendship, to have a RIRO interchange with SR 189. They continue southeast to have a Quadrant interchange with SR 152 just before entering Alamo. US 412/SR 20 then have a parclo interchange with SR 188 before having a partial interchange with N Cavalier Drive. US 412/SR 20 then leave Alamo and continue southeast through farmland and have a Quadrant interchange with SR 88 before entering Bells and coming to a Cloverleaf interchange with US 70A/US 79/SR 76, just after bypassing downtown to the northern and eastern sides. It then has a parclo interchange with the eastern end of SR 88 before leaving Bells and continuing southeast through farmland to cross into Madison County.
Madison County
The highway then enters some hills before entering Jackson and coming to a diamond interchange with Country Club Lane, approximately from its parclo interchange with I-40. At I-40, SR 20 remains hidden and follows US 412 Bus. through the city while US 412 is concurrent with I-40 between exits 79 and 87. US 412 Bus./SR 20 follows Hollywood Drive as a four-lane undivided highway, passing by several businesses and homes before coming to an intersection with North Parkway, where they make a left to follow that street through several neighborhoods just before an intersection with US 45 Byp./SR 186. US 412 Bus./SR 20 continues east through several more neighborhoods to an intersection with US 45/SR 5 in the middle of a major shopping district. They then continue through more neighborhoods and pass through an industrial area before coming to an intersection with US 70/SR 1, where they make a left to become concurrent with that highway. They then travel through some suburbs and pass by an industrial park before coming to an intersection with US 412, where US 412 Bus. ends and SR 20 splits from US 70/SR 1 as it becomes concurrent with US 412 again as the pair exit Jackson as a two-lane highway and continue east. The entire route of SR 20 through the city of Jackson is at least four lanes wide. US 412/SR 20 continue east through farmland to the community of Springbrook, where it widens to a four-lane divided highway and has an intersection with SR 152 just before entering Henderson County.
Henderson County
US 412/SR 20 continue east through countryside and farmland and pass through Crucifer before narrowing to a 4-lane undivided shortly before entering Lexington. They then pass by several businesses and becomes concurrent with SR 104 before crossing over the Beech River, passing just south of the dam for Beech Lake. The highway continues past several more businesses before entering downtown and coming to an intersection with SR 22, where SR 104 splits off and goes southeast. US 412/SR 20 continues through some neighborhoods before leaving Lexington and continuing east through farmland and countryside as a 4-lane divided highway. The highway then narrows to a 4-lane undivided highway before passing through the communities of Chesterfield, where it has an intersection with SR 114, and Darden. The highway then crosses into Decatur County.
Decatur County
US 412/SR 20 then have an intersection with SR 202 before continuing east and entering Parsons. They pass by a neighborhood before narrowing to 2-lanes and entering downtown, where it has an intersection with US 641/SR 69. They continue through downtown before leaving Parsons as a 2-lane highway. US 412/SR 20 continue through largely wooded countryside before coming to Perryville and becoming concurrent with SR 100, just before crossing a bridge over the Tennessee River and entering Perry County.
Perry County
The highway has an intersection SR 438 just south of Mousetail Landing State Park before passing through some hills before entering Linden. They pass through downtown before having an intersection with SR 13. The highway then leaves Linden after crossing a bridge over the Buffalo River and continues east to the community of Chestnut Grove, where SR 100 splits off at a Y-Intersection and goes northeast. They then pass through hilly and wooded areas as they cross into Lewis County.
Lewis County
US 412/SR 20 continues east through hilly and wooded terrain to enter farmland, just before entering the town of Hohenwald. The highway passes by some homes and businesses before entering downtown and coming to an intersection with SR 48 and SR 99, where SR 20 leaves US 412 and becomes signed for the first time as it goes south along SR 48/SR 99. A short distance later after leaving downtown, SR 20 and SR 99 split off from SR 48 and turn east before curving to the south and to a Y-intersection where SR 99 splits off and turns southwest. SR 20 then curves eastward and leaves Hohenwald as a lone 2-lane highway. It then passes through farmland and wooded areas and has an interchange with the Natchez Trace Parkway before entering Lawrence County.
Lawrence and Maury Counties
SR 20 continues east through farmland before entering Summertown and becoming concurrent with SR 240 via a Y-intersection. They then pass through the center of town before SR 240 splits off and SR 20 crosses into Maury County, shortly before the highway comes to an end at an intersection with US 43/SR 6.
Prior to 1981 and dating back as far as 1958, SR 20 at its western end extended beyond Dyersburg following what is now SR 210, US 51, SR 211 and SR 104 to Heloise, Tennessee on the Mississippi River in extreme western Dyer County.
Future
currently has plans to expand existing SR 20 from two-lane to four-lane or completely reconstruct the route on a new alignment from Interstate 40 in Madison County to the Tennessee River near Perryville.
