Jason Isbell and The 400 Unit is an American country group from Muscle Shoals, Alabama, led by former Drive-By Truckers singer and guitarist Jason Isbell. The line-up includes Isbell on vocals and guitar, Sadler Vaden on guitar, Jimbo Hart on bass, Derry deBorja on keys and accordion, and Chad Gamble on drums. Amanda Shires appears regularly as a session player and occasional touring member, contributing fiddle and vocals.

The group takes its name from the informal term for the psychiatric ward at Eliza Coffee Memorial Hospital in Florence, Alabama. It is active from 2009 onwards and is associated with country, Americana and Southern rock. The band is credited on five of Isbell’s studio albums: Jason Isbell and the 400 Unit (2009), Here We Rest (2011), The Nashville Sound (2017), Reunions (2020) and Weathervanes (2023).

Members of the group also appear on Isbell’s earlier solo albums, including Sirens of the Ditch (2007), Southeastern (2013) and Something More Than Free (2015). The band tours regularly in the United States and internationally, and also plays an annual week of shows at Nashville’s Ryman Auditorium, with Shires and a range of country, folk and soul support acts.