The vehicle of K. Sivanesan, a member of parliament for the pro-rebel Tamil National Alliance (TNA), was hit by a mine in a guerrilla-controlled part of the north of the island, his fellow lawmaker Mavai Senathirajah said.
The blast was at Mallavi, 35 kilometres (21 miles) north of the frontline post of Omanthai, officials said.
"The government knew Mr. Sivanesan was crossing the Omanthai checkpoint and returning to his home after attending parliament," Senathirajah said.
The rebel Liberation Tigers of Tamil Eelam said the attack was carried out by a "deep penetration unit" of the Sri Lankan army.
Tiger rebels have regularly accused government forces of carrying out bomb blasts inside territory controlled by them, which the military denies.
The reported killing came as security forces said they had captured more guerrilla territory in the northwest after weeks of heavy fighting.
So far this year, the defence ministry has reported that security forces have killed at lea