Geagea, 61, was the only leader from the 1975-1990 civil war ever jailed, but was released under an amnesty in 2005 after Syria withdrew from Lebanon, ending 30 years of domination over its tiny neighbour.
Syria ended its military and political dominance under popular Lebanese and international pressure, but it continues to exert significant influence through allies, mainly Hezbollah.
Geagea is one of Hezbollah's key opponents in Lebanon, and has repeatedly demanded that the movement, which never disarmed after the 15-year civil war, hand in its weapons.
Last month, Geagea's Lebanese Forces refused to join a new government finally formed after 10 months of political wrangling, citing Hezbollah's involvement in Syria and its controversial arsenal.
Lebanese Forces deputy Georges Edwan announced Geagea's candidacy, saying the party's decision to back him is based on an "urgent need for radical change now".
The war in Syria has exacerbated sectarian tensions in Lebanon as the conflict has spilled over into the Mediterranean country.
The situation has been further strained as the number of refugees from Syria has hit the one million mark in Lebanon, a country of fragile resources and a native population of just four million.
Geagea hopes to take over from President Michel Sleiman whose six-year term ends on May 25 and who was the first head of state voted in after Syria withdrew in 2005.
Under the constitution, parliament chooses a president within a two-month period before the end of the incumbent's term.