Milanovic defeated a candidate from the ruling conservative party in a runoff vote, near-complete official results show.
He won nearly 74 per cent of the vote compared with his challenger Dragan Primorac, who gained around 26 per cent, according to the results released by Croatia's state election authorities on Sunday, after more than 70 per cent of the ballots were counted.
The result presents a major boost for Milanovic, who is a critic of Western military support for Ukraine in its war against Russia.
Milanovic, 58, is the most popular politician in Croatia, and is sometimes compared to US president-elect Donald Trump for his combative style of communication with political opponents.
His triumph also sets the stage for a continued confrontation with Croatia's powerful Prime Minister Andrej Plenkovic. Sparring between the two during Milanovic's first term in office has marked Croatia's politics.
Milanovic had comfortably won also in the first round of voting on December 29, leaving Primorac, a forensic scientist who had unsuccessfully run for president previously, and six other candidates far behind.
The runoff between the top two contenders was necessary because Milanovic fell short of securing 50 per cent of the vote by just 5000 votes, while Primorac trailed far behind with 19 per cent.
The election was held as the European Union and NATO member country of 3.8 million people struggles with biting inflation, corruption scandals and a labour shortage.
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