The captain of the plane, Flight 319, "declared an emergency" after detecting the smell and landed without incident at BWI, Southwest officials said.
The plan taxied to the gate "under its own power," the airline said.
Local emergency units had responded to the tarmac for a "precautionary alert" of an inbound plane under a declared emergency.
The 113 passengers on board were taken to Boston on a separate flight and were expected to arrive about two hours behind schedule, Southwest said.
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.
The incident had a "minor impact" on airline operations, and security checkpoints were quickly back to normal operations, said Whitney Kidd, the BWI spokeswoman.