Chattanooga ( CHAT-ə-NOO-gə) is a city in and the county seat of Hamilton County, Tennessee, United States. Located along the Tennessee River bordering Georgia, it also extends into Marion County on its western end. With a population of 181,099 in 2020, it is Tennessee's fourth-largest city and one of the two principal cities of East Tennessee, along with Knoxville. It anchors the Chattanooga metropolitan area, Tennessee's fourth-largest metropolitan statistical area, as well as a larger three-state area that includes Southeast Tennessee, Northwest Georgia, and Northeast Alabama.

Chattanooga was a crucial city during the American Civil War, due to the multiple railroads that converge there. After the war, the railroads allowed for the city to grow into one of the Southeastern United States' largest heavy industrial hubs. Today, major industry that drives the economy includes automotive, advanced manufacturing, food and beverage production, healthcare, insurance, tourism, and back office and corporate headquarters. Chattanooga remains a transit hub in the present day, served by multiple Interstate highways and railroad lines. It is 118 miles (190 km) northwest of Atlanta, Georgia, 112 miles (180 km) southwest of Knoxville, Tennessee, 134 miles (216 km) southeast of Nashville, Tennessee, 102 miles (164 km) east-northeast of Huntsville, Alabama, and 147 miles (237 km) northeast of Birmingham, Alabama.

Divided by the Tennessee River, Chattanooga is at the transition between the ridge-and-valley Appalachians and the Cumberland Plateau, both of which are part of the larger Appalachian Mountains. Its official nickname is the "Scenic City", alluding to the surrounding mountains, ridges, and valleys. Unofficial nicknames include "River City", "Chatt", "Nooga", "Chattown", and "Gig City", the latter a reference to its claims that it has the fastest internet service in the Western Hemisphere.

Chattanooga is internationally known from the 1941 hit song "Chattanooga Choo Choo" by Glenn Miller and his orchestra. It is home to the University of Tennessee at Chattanooga (UTC) and Chattanooga State Community College.