The run-off is due to take place in early January
Opposition Social Democrat leader Ivo Josipovic will face independent Milan Bandic in a run-off for the Croatian presidency, electoral officials say.
Mr Josipovic gained more votes than his rival, but not more than the 50% needed to secure outright victory, Croatia's electoral commission confirmed.
The second-round of voting is due to take place on 10 January.
Voters are concerned about corruption and the economy as Croatia aims to join the EU, observers say.
With nearly all votes counted, Mr Josipovic had won 32.4% of the vote with Mr Bandic on 14.8%.
Mr Josipovic, a relative newcomer, has had a career untarnished by corruption, but analysts say he could be seen as lacking the charisma needed to take office.
Opinion polls had predicted he would be closely followed by Mr Bandic.
The Croatian presidency is a largely ceremonial role. The president has a say in foreign policy, security and defence but no power to veto legislation.
Popular centrist Stipe Mesic is stepping down in February after serving two five-year terms.
Croatia joined Nato in April, but EU membership has been delayed by a border dispute with Slovenia dating back to the break-up of Yugoslavia.