The pilot of flight 448, bound for Los Angeles, was alerted by the sound of banging "from beneath the aircraft", an Alaska Airlines statement said.
Once back on the ground, the baggage handler emerged from the pressurised hold, saying he had fallen asleep.
He "appeared OK" but went to hospital as a precaution, the airline said.
Alaska Airlines later said the worker had passed a drugs test and been released from hospital.
The Boeing 737 aircraft had taken off from Seattle with 170 passengers on board, and was airborne for 14 minutes.
According to Alaska Airlines, the worker`s team leader had noticed that the man was missing before the plane took off.
The team leader had called into the cargo hold and rung the worker`s mobile phone but did not get an answer.
"His co-workers believed he finished his shift and went home," the airline said.
The airline said the worker, employed by Menzies Aviation, started work at 05:00 local time and was due off at 14:30, but fell asleep in the cargo hold. The compartment was pressurised - so survivable at altitude - and was temperature-controlled.
One passenger, Marty Collins, told a local Seattle TV station that passengers had not heard the banging.
She said: "Nobody on the plane heard anything like that, nobody knew why we were turning around. They just said we were fine and we weren`t in any danger."
Later, Ms Collins said passengers were told "there was someone in the cargo hold and he`s been escorted off and taken away".
The flight later left for Los Angeles.
Alaska Airlines plane `flew with worker trapped in cargo hold`
14 April 2015
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An Alaska Airlines plane declared an emergency and made a priority landing in Seattle after taking off with a worker trapped in the cargo hold.
