Grabar-Kitarovic, a 46-year-old ex-foreign minister and former NATO official, had 51.4 percent of the vote, according to exit polls released by state-run HRT television after today's run-off.
Her rival, centre-left incumbent Ivo Josipovic, garnered 48.6 percent of the vote, showed the exit polls released by the Ipsos Puls agency.
The first partial official results were to be released by the electoral commission around 2030 GMT.
Should they confirm the exit polls, Grabar-Kitarovic, who ran on behalf of the main opposition HDZ party, will be the first Croatian woman to take the helm of the European Union's newest member.
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The election for the largely ceremonial post was held as Croatia, which became the EU's 28th member in July 2013, grappled with a deep economic crisis.
Josipovic, a popular former law professor and classical music composer who belongs to the Social Democrats (SDP), the main force in the ruling coalition, had also pledged to pump new life into the economy.
Although presidential powers are limited in Croatia, today's vote is seen as a key test for parliamentary elections later this year in which Grabar-Kitarovic's HDZ is likely to make significant gains.