Ali Yusuf Khalil, who was in his 70s, was chairman of the parliament's foreign affairs committee and represented Lebanon's southern area.
A former cabinet minister, Khalil supported the government during the current crisis over Syria's troops in Lebanon and the investigation of former prime minister Rafiq al-Hariri's assassination.
Khalil's son and driver survived the late Sunday crash on the coastal highway near Khalda, south of Beirut.
A police officer said the driver apparently lost control on the wet road and skidded into the concrete and steel barrier dividing the two sides of the highway. The car was wrecked.
A Shia Muslim, Khalil belonged to the parliamentary bloc of Speaker Nabih Barri. He served many terms in parliament, having first been elected before Lebanon's civil war of 1975-90.
The parliament is divided equally between Muslim and Christian legislators.
The parliament's mandate ends on 31 May.
Lebanon has been without a government for the past four weeks as pro-government and opposition legislators have failed to agree on the formation of a cabinet to oversee the holding of elections.
The government fell on 28 February after massive demonstrations against Syria's military presence and the killing of al-Hariri in a bomb attack two weeks earlier.