Ivo Sanader, in office from 2003 to 2009, was convicted of taking millions of dollars in bribes from a Hungarian energy company and an Austrian bank.
The former prime minister denied wrongdoing at his trial.
Croatia, which hopes to join the EU in July of next year, is under pressure to tackle widespread corruption.
Correspondents say the EU is taking a harder line with the Balkan state than with Romania and Bulgaria, which were allowed to join the bloc despite struggling to address their problems with corruption.
Ivo Sanader, 59, is the most senior official to have been convicted of corruption in Croatia.
He was found guilty of accepting a bribe of $12.8 million (10 million euros) from the Hungarian oil company MOL in return for securing it controlling rights in Croatia’s state oil company Ina.
Unless Tuesday’s verdict is overturned on appeal, Croatia may review MOL’s shareholder agreement with Ina, Reuters news agency says.
In 1995, when Ivo Sanader was a deputy foreign minister, he received $695,000 in bribes for a credit deal with the Hypo Alpe Adria Group, which gave the Austrian bank a leading position in Croatia.
At the time, Croatia was still fighting its war of independence from Yugoslavia, meaning it had trouble accessing the international markets.
Prosecutors described Ivo Sanader’s action as “war profiteering”.
Ivo Sanader argued that the case against him was politically motivated.
He is also on trial, separately, for allegedly creating slush funds for his political party, the conservative Croatian Democratic Union (HDZ), by skimming off profits from state companies and manipulating public tenders.
The HDZ ruled the country for eight years until its defeat in elections in December 2011.
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