The flight was diverted and landed at a Yuma, Ariz., military base.
The National Transportation Safety Board (NTSB) is investigating the incident.
The rupture caused the plane to descend suddenly and a drop in cabin pressure.
Passengers said a large hole tore the top of the plane.
No one was injured. One-hundred-eighteen people were onboard.
Sudden drop in pressure forces emergency landing
Report from the Associated Press
Federal officials said a "fuselage rupture" forced a Southwest Airlines flight to make an emergency landing Friday in the Arizona desert city of Yuma, and passengers described a large hole at the top of the plane.
"It's at the top of the plane, right up above where you store your luggage," passenger Brenda Reese told The Associated Press in a telephone interview. "The panel's not completely off. It's like ripped down, but you can see completely outside... When you look up through the panel, you can see the sky."
The cause of the hole was not immediately known.
Reese said the plane had just left Phoenix Sky Harbor International Airport for Sacramento, Calif., when she awoke after hearing a "gunshot-like sound." She said oxygen masks then dropped for passengers and flight attendants as the plane dove.
The plane, which was carrying 118 people, landed at a military base in Yuma without any injuries reported, according to the airline. Reese said a flight attendant fell and injured his nose, and said some people "were passing out because they weren't getting the oxygen."
The National Transportation Safety Board said an "in-flight fuselage rupture" led to the sudden descent and drop in cabin pressure aboard the Boeing 737.
Ian Gregor, a Federal Aviation Administration spokesman in Los Angeles, said the pilot "made a rapid, controlled descent from 36,000 feet to 11,000 feet altitude after the incident occurred."
"It dropped pretty quick," said Reese, who provided cellphone photographs of the cabin damage. The pictures showed a panel hanging open in a section above the plane's middle aisle, with a hole of about six feet long.
Julie O'Donnell, an aviation safety spokeswoman for Seattle-based Boeing Commercial Airplanes, confirmed there was "a hole in the fuselage and a depressurization event" but declined to speculate on what caused the incident.
Reese said there was "no real panic" among the passengers, who applauded the pilot after he emerged from the cockpit following the emergency landing at Yuma Marine Corps Air Station/International Airport, some 150 miles southwest of Phoenix and about 40 minutes after takeoff from Sky Harbor.
Steve Dupre, a spokesman for the FBI in Sacramento, said the agency was not investigating because "it appears to be a mechanical issue."
Gregor said an FAA inspector from Phoenix was en route to Yuma. The NTSB said it also was sending a crew to Yuma.
Gina Swankie, a spokeswoman for Sacramento International Airport, said passengers would be put on another flight to Sacramento later Friday.
"I fly a lot. This is the first time I ever had something like this happen," said Reese, a 37-year-old single mother of three who is vice president for a clinical research organization. "I just want to get home and hold my kids."
Holes in aircrafts can be caused by metal fatigue or lightning. The National Weather Service said the weather was clear from the Phoenix area to the California border on Friday afternoon.
In October 2010, a cabin lost air pressure when a hole ripped open in the fuselage of a Boston-bound American Airlines flight from Miami, also forcing an emergency landing.
In 1988, a Boeing 737 blew open at 24,000 feet when a 20-foot section of the aircraft's upper fuselage ripped off. An Aloha Airlines flight attendant was sucked out of the jet and killed, and 61 passengers were injured.
Associated Press writers Don Thompson and Adam Weintraub contributed to this report from Sacramento, Calif.