The diverted plane was taken out of service "for inspection and any repairs," Southwest said.
A BWI spokeswoman said the landing occurred "without incident."
Also, on Thursday afternoon, the security checkpoints for Concourses A, B and C at BWI were closed for about 10 minutes after alarms began sounding in the terminal.
According to the Transportation Security Administration, a passenger had just exited the secure portion of the airport behind security about 2:20 p.m. when he turned back and re-entered the area through the exit.
This automatically triggered an alarm. The individual, who did not speak English, was referred to the Maryland Transportation Authority for questioning.