US Alaska Airlines Flight 915, which was scheduled for departure on Tuesday from Oakland, California to Portland, Washington, was canceled after passengers boarded the plane.
Most of the airline's 110 passengers were directed to another flight to Portland later.
"Our flight to Portland is delayed because when they opened the door on arrival, a rat jumped into the plane. There are four persons trying to locate it," a passenger named Matt Gough tweeted.
Another passenger said: "AlaskaAir flight got cancelled in Oakland due to rat on plane."
Alaska Airlines said in a statement that the plane "is currently out of service."
"It will be returned to operations once it's certified rodent-free by a professional exterminator. The aircraft will also be thoroughly inspected to ensure no damage has been done," said the airline.
Alaska Airlines, a major US air carrier founded in 1932 and headquartered in Seattle, Washington, has flights to more than 100 destinations in the US and beyond.