The gas cylinders spilled across the road, but none of the metal canisters exploded or caught fire.
Both lanes into the tunnel were closed for more than two hours, leading to long tailbacks of vehicles heading both directions, northbound and southbound.
Officers from Phuket City Police and rescue workers swiftly arrived at the scene.
A passenger in the truck cab sustained minor injuries and was promptly taken to Mission Phuket Hospital.
The driver of the truck, Dawit Kongsumboon, 28, uninjured, told officers that he was driving from Mai Khao in Thalang to deliver gas to Ao Makham.
While driving into the tunnel the truck began to increase speed, and he struck the guardrail, causing the truck to overturn, he said.
Of the 30 gas cylinders that spilled from the back of the truck, seven were found to be leaking. The rescue workers doused the truck and had the gas cylinder safely removed before reopening the road for normal traffic flow.
Phuket Provincial Police are still investigating the incident, awaiting footage from surveillance cameras within the underpass and assessing the extent of the damage caused by the accident. Further legal action will be taken based on the investigation’s findings, police said.