U.S. 23 extends just 37.9 miles south into the Sunshine State, mostly along shared alignments with U.S. 1 and 301. The route joins the Nassau County town of Callahan with Jacksonville to the south and Folkston, Georgia to the north. The 2.549 mile segment of U.S. 23 along New Kings Road and Kings Road through Grand Park and Mid Westside in Jacksonville represents the lone stand alone portion of the route in Florida. Prior to 2006, U.S. 23 ran solo from I-95 to U.S. 1/17 (Main / Ocean Streets) as well. That changed with the realignment of U.S. 17 to overlap with U.S. 23 along the couplet of State and Union Streets.
North into Georgia, U.S. 23 and SR 23 briefly overlap through Folkston. Georgia Route 23 previously formed a multi state route with Florida State Road 23 leading south to Macclenny, Lake Butler and Gainesville. It was renumbered as part of multi state route 121 between Lebanon Station, Florida and Rock Hill, South Carolina. SR 23 in Florida was eventually reused for the First Coast Expressway.