Officers say a woman screaming her boyfriend had been stabbed ran off a SkyTrain at Edmonds Station between Burnaby and New Westminster.
Police say the couple got in an argument with three men who were also on board.
Officers say over the course of the argument a knife was pulled and the woman's boyfriend was stabbed in the arm.
Three men left the train, two of whom were arrested but later released.
The suspect is described as a white man between 20 and 25 years old, about five feet, eight inches tall, and 150 pounds with wavy brown hair cut shorter on one side.
The victim, a 40-year-old man from Surrey, received five stitches in hospital and was later released.