"Aridi was killed in the explosion in Baysur, near Aley," the security official said, while the army spokesman said three other people were injured.
"The initial probe indicates that the bomb was placed inside his Mercedes," another security official said.
Aridi, who was in his 50s, was alone in his vehicle when the bomb detonated, the officer said.
There have been no claims of responsibility for the attack, Al Jazeera's correspondent in Beirut reported.
Druze adviser
Aridi was a prominent adviser to Talal Arslan, a prominent Druze politician whose Democratic Party has close links to neighbouring Syria.
Arslan is a leading opponent of Walid Jumblatt, de facto leader of the Druze community and head of the Progressive Socialist Party (PSP).
Jumblatt has paid a visit to Aridi's family to offer his condolences, Sherine Tadros, Al Jazeera's correspondent in Beirut, reported.
The bombing comes as Lebanon's political parties get ready to hold a national dialogue meeting next week in an attempt to resolve outstanding difficulties.
The talks come four months after members of the Western-backed majority bloc in Lebanon's parliament agreed to form a national unity government with the Syrian-supported political opposition, which is led by Hezbollah, a Shia Muslim organisation.
The agreement between the parties led to Michel Sleiman, the chief of the Lebanese army, being elected as Lebanon's president.
Wednesday's attack is the first deadly car bombing since January, when a security official investigating a series of political killings blamed on Syria was killed.
Aridi is the first opposition politician to be assassinated since a string of politically motivated attacks against members of the parliamentary majority bloc began in late 2004.