The 53-year-old former prime minister is poised to win 29.58 per cent of the vote, according to a poll by the national television.
Milanovic is trailed by sitting President Kolinda Grabar-Kitarovic with 26.38 per cent and independent candidate Miroslav Skoro with 24.1 per cent.
The remaining eight candidates won a combined 6 per cent of the vote. If no candidate secures over 50 per cent of the vote the election will go into a second round.