The bells of St Mark's Church chimed and a choir sang the Croatian national anthem, "Our Beautiful Homeland", before the swearing-in ceremony.
Mesic took the oath before Constitutional Court President Petar Klaric and judges of the court, saying and signing the text of the oath written in the Constitution.
Mesic won his second term in an election run-off on January 16 by defeating the candidate of the ruling Croatian Democratic Union (HDZ) party, Jadranka Kosor.