The September 2009 Southeastern United States floods were a group of floods that affected several counties throughout northern Georgia, Tennessee, Alabama, Mississippi, and Arkansas. The worst flooding occurred across the Atlanta metropolitan area. Continuous rain, spawned by moisture pulled from the Gulf of Mexico, fell faster than the local watersheds could drain the runoff.

Initial damages from around the state were estimated at $250 million. On September 26, Georgia Insurance Commissioner John Oxendine raised the estimated cost to $500 million with the potential for it to rise. Some 20,000 homes, businesses and other buildings received major damage and 17 Georgia counties received Federal Disaster Declarations. The flood is blamed for at least ten deaths.

The floods were historic, breaking records that went back more than a century in some locations. The Chattahoochee River, the largest river in the region, measured water levels at a 500-year flood level.

The floods occurred not long after the end of the 2006–2008 Southeastern United States drought. Rain began falling on the Atlanta area on September 15, 2009, with the National Weather Service (NWS) reporting only 0.04 inches that day at the Hartsfield–Jackson Atlanta International Airport. Additional rain fell throughout the week, with only a trace amount recorded for September 18. However, a large rain event began to inundate the area on September 19. The official NWS monitoring station at the Atlanta airport recorded 3.70 inches (94 mm) of rainfall from daybreak to 8pm (more than doubling the previous record for rainfall on that date), while outlying monitoring stations recorded 5 inches (130 mm) of rainfall in a 13-hour period. Flooding began in one neighborhood that day, with the remainder of the area placed under a flash flood watch for the rest of the weekend.

Hundreds of people were rescued by boat from their homes, and at least ten people died, mostly in their cars by driving where water crossed the road, which motorists were repeatedly warned against on local radio and TV. The American Red Cross started emergency shelters in each county affected by the floods. Most Atlanta area school districts were closed September 21 due to floodwaters and road closures which made school bus routes longer and more dangerous.

The Chattahoochee rose to the highest levels since the Buford Dam was built. Water levels along the river rose over the 0.2 percent chance exceedence (500-year) flood at the gage location. The River reached its second-highest level ever in Vinings at Paces Ferry Road, and would have surpassed its 1916 record were it not for the impoundment built in the 1950s. Along the river in both Vinings and Roswell, a one percent chance exceedence (100-year flood) level was measured.

Peachtree Creek, a tributary of the Chattahoochee, topped its stream gauge and the bridge itself at Northside Drive, but remained just below its 1916 record. However, Nancy Creek–another Chattahoochee River tributary–did reach a record level, destroying the Peachtree-Dunwoody Road bridge (which would reopen on March 23, 2010).

The western side of the Interstate 285 beltway crosses the Chattahoochee River. This section of the interstate was completely underwater for several days. Many of the roller coasters and rides at Six Flags Over Georgia were partly underwater, with at least 80% of Great American Scream Machine submerged by the Chattahoochee River.