Southwest spokeswoman Olga Romero says a passenger on the scheduled 6:15 a.m. Monday flight from Houston's Hobby Airport found the box cutter and turned it over to the crew.
The plane's captain alerted airport security. All passengers were taken off the plane and re-screened.
The plane was searched, but no other hazards were found.
Romero says the passengers have since been rebooked and arrived in Dallas.
Romero says they are still investigating the incident and how the box cutter ended up in the overhead bin in the first place. There were 96 passengers and five crew members on board.
The Transportation Security Administration didn't immediately return a phone call seeking comment.
The Associated Press contributed to this report