SH-45 begins at an intersection with SH-78 before crossing the Snake River and heading northeast.

The highway continues north into the city of Nampa as 12th Avenue Road, passing by Mercy Medical Center and Northwest Nazarene University.

Then, SH-45 turns northeast on 12th Avenue before forming a one-way couplet with 11th Avenue before ending at 2nd Street South.

The city government began studying a realignment in 2021 that would use one of several existing roads that lead to I-84 from the south side of downtown Nampa.

[3][4] State Highway 45 Connector (SH-45 Conn.) is a short route connecting SH-78 and SH-45.