Slovakia's diplomat Miroslav Lajcak is taking over as the international High Representative for Bosnia-Hercegovina.
Mr Lajcak aims to strengthen Bosnia's central government
Mr Lajcak has said his first task will be to help end the country's political deadlock and revive the economy - 12 years after the end of the Bosnian war.
The planned handover of power to local lawmakers in June 2007 has been put back as Bosnia is still deeply divided along ethnic lines.
Mr Lajcak is replacing Christian Schwarz-Schilling, a German diplomat.
Mr Schwarz-Schilling Bosnia's international administrator since early 2006, but critics say he has failed to make politicians from rival ethnic groups work together to agree core reforms.
This was partly due to the lack of reform but also because of fears that any Western recognition of independence for ethnic Albanians in Kosovo could trigger a reaction among Bosnian Serbs.
The High Representative - a post set up under the 1995 Dayton Peace Agreement that ended the Bosnian war - has the power to dismiss Bosnia's top politicians and officials.
Mr Schwarz-Schilling has served since January 2006
Mr Lajcak has said that his first challenge will be "stabilisation of the atmosphere".
He has said that his priority will be unifying the various ethnic police forces and constitutional reform that would strengthen the central government - issues on which lawmakers are deadlocked.
When Mr Schwarz-Schilling took office, he said that his goal was to hand power back to Bosnian politicians.
But ethnic divisions have continued to dominate politics, with leaders of Bosnia's Serbs, Muslims and Croats failing to agree on key reforms required for Bosnia's entry in the European Union.
"Certain key political leaders... have been irresponsible in their actions, putting their own ambitions ahead of the well-being of all the citizens of Bosnia-Hercegovina," Mr Schwarz-Schilling said at a press conference in Sarajevo on Thursday.
But, says the BBC's Nicholas Walton in Sarajevo, many Bosnians also blame the High Representative for being too weak and ineffectual to force these reforms through.
Power is due to be returned to Bosnian politicians in mid-2008.