It serves to connect the towns of Gruetli-Laager and Palmer to Cagle and the Savage Gulf State Natural Area portion of South Cumberland State Park.

It heads northeasterly through farmland and rural areas to cross a bridge over the Collins River before passing by the Savage Gulf State Natural Area portion of South Cumberland State Park.

The highway then curves to the east and crosses into Sequatchie County.

SR 399 then goes through remote wooded areas for the next several miles before entering farmland and the community of Cagle to come to an end at an intersection with SR 8/SR 111.

The entire route of SR 399 is a two-lane rural highway and lies entirely atop the Cumberland Plateau.