Chattanooga (/ˌtʃætəˈnuːɡə/ CHAT-ə-NOO-gə) is a city in Hamilton County, Tennessee, United States, and its county seat. It is located along the Tennessee River and borders Georgia to the south. With a population of 181,099 in 2020, it is Tennessee's fourth-most populous 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 southeastern Tennessee, northwestern Georgia, and northeastern 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 referring to Chattanooga's claim of having 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.

The first inhabitants of the Chattanooga area were Native Americans. Sites dating back to the Upper Paleolithic period (c. 10,000 BCE) show continuous human occupation through the Archaic, Woodland, Mississippian/Muskogean/Yuchi (900–1714 CE), and Cherokee (1776–1838) periods. The Chickamauga Mound near the mouth of the Chickamauga Creek is the oldest (c. 750 CE) remaining visible art in Chattanooga.

The Citico town and mound site was the most significant Mississippian/Muscogee landmark in Chattanooga up to 1915. The first part of the name "Chattanooga" derives from the Muskogean word cvto /chắtȯ/ – 'rock'. The latter may be derived from a regional suffix -nuga meaning dwelling or dwelling place. It is also believed to be derived from the Creek Indian word Chat-to-to-noog-gee, meaning 'rock rising to a point', which is speculated to be a reference to Lookout Mountain.

The earliest Cherokee occupation of the area dates from 1776, when Dragging Canoe separated himself from the main tribe to establish resistance to European settlement during the Cherokee–American wars. In 1816 John Ross, who later became Principal Chief, established Ross's Landing. Located along what is now Broad Street, it became one of the centers of Cherokee Nation settlement, which also extended into Georgia and Alabama.

In 1838, the U.S. government forced the Cherokees, along with other Native Americans, to relocate to the area designated as Indian Territory, in what is now the state of Oklahoma. Their journey west became known as the "Trail of Tears" for their exile and fatalities along the way. The U.S. Army used Ross's Landing as the site of one of three large internment camps, or "emigration depots", where Native Americans were held before the journey on the Trail of Tears.