The tight race saw the anti-immigration right-wing Finns Party come in second place with 17.5%, while their coalition partners the National Coalition Party, and the Center Party scored 17% and 13.8% respectively, Italehti reported.
All centre-left and left-wing parties saw an increase in their vote, with the Green League coming in fifth with 11.5% of the vote, followed by the Left Alliance at 8.2%.
Between them, the three left-leaning parties should return 76 lawmakers, according to Italehti's estimates, short of a majority in Finland's 200-seat parliament, meaning a governing coalition will likely have to include one of the main centrist parties.
SDP Chairman Antti Rinne, as leader of the largest party, will have first shot at forming a government, and at becoming the first left-wing prime minister in Finland since 2003.
In a speech Sunday, former union boss Rinne thanked party members for their hard campaign work.
"For the first time since 1999 we are the largest party in Finland ... SDP is the prime minister's party," he told supporters in central Helsinki, Reuters reported.
CNN's James Griffiths contributed to this report.