Phanuwat Nakprakorb, 27, a nurse at a hospital in Nonthaburi, died at the scene.
Police said the accident happened at 6.45am in the vehicle tunnel under the Pak Kret Intersection on Tiwanon Road.
Phanuwat was heading towards the Kaerai Intersection when he entered the tunnel, police said. He was hurled across the lane barrier when his bike fell to one side at a curve and he landed on the road in the opposite lane, about 20 metres from his bike.