The Western African former French colony has a two-term limit for the presidency, although the high court ruled in January President Abdoulaye Wade could run for a third time, Voice of America reported.
His main opponent is Macky Sall, who ran Wade's campaign in 2007 and also served as federal mining minister and prime minister.
Wade, 85, campaigned on the platform that he hasn't had enough time to complete critical projects.
Wade had 35 percent support to Sall's 27 percent in the first round of voting Feb. 25, and the narrow spread led to Sunday's runoff vote.
Senegal is considered one of Africa's most stable democracies, although riots shook the capital of Dakar in January when the high court allowed Wade to run for a third term.