Stimac offered his resignation immediately after Tuesday's 2-0 defeat to Scotland at Hampden Park and the Croatian football federation (HNS) on Wednesday confirmed it had decided to part company with the former Derby and West Ham defender.
The loss to Scotland left Croatia with just one point from their final four World Cup qualifiers and meant they had to settle for a play-off spot after finishing in second place in Group A behind Belgium.
The HNS also announced in a statement on its official website that Under-21 boss Nico Kovac has been appointed as interim head coach.
The statement read: "At today's meeting of the Croatian football federation's executive board the series of poor results and matches of the senior team were discussed.
"At the suggestion of president Davor Suker, the executive board made the following decisions:
"First: Igor Stimac is relieved of his duties as coach of the senior team and his contract terminated.
"Second: Nico Kovac has been appointed as acting senior team coach with the intention of appointing a coach for the next qualifying cycle, which is not conditional on the upcoming World Cup qualifiers."
Croatia had won five of their opening six qualifiers in Group A, while drawing the other away to Belgium, but then badly lost their way with automatic qualification within their sights.
A 1-0 home defeat to Scotland started the rot, and Croatia followed that with a 1-1 draw in Serbia, a 2-1 loss to Belgium in Zagreb last Friday and culminated in Tuesday's defeat in Glasgow.
The 46-year-old Stimac, who had been in charge since July 2012, said after the latest loss: "It is a bad defeat. I offered my mandate (resignation) to the president of the Croatian football federation because of the latest results of our team, because everything that surrounds us.
"Never mind that we qualified for the play-offs, I want to show some morale, certain responsibility and if there is anyone in the federation that does not believe in my work I want to give them the chance to change.
"They will have a meeting to discuss it."
The new man in the hotseat is 42-year-old former national team captain Kovac, who played 83 times for Croatia while also representing the likes of Bayern Munich, Hamburg and Hertha Berlin in Germany.
He joined Red Bull Salzburg in 2009 and later became an assistant coach at the Austrian side before taking on the Croatia Under-21 job earlier this year.
Suker told reporters on Wednesday: "It's hard for Igor Stimac, a former team-mate of mine, but we had to look at the reputation of the team.
"We leave the future to Niko Kovac and hopefully he will take us to the World Cup. We turned to him as a former team-mate, a team captain and one who has made a great impression in charge of the Under-21 team."
Croatia will find out their World Cup play-off opponents in Monday's draw.
Fornalik, 50, took charge of Poland following the departure of Franciszek Smuda after the side's early exit from Euro 2012, agreeing a deal until the end of the World Cup qualifying campaign that would have been renewed had qualification to Brazil been achieved.
However, Poland saw their hopes of reaching the finals ended on Friday with defeat in Ukraine, while a 2-0 loss against England on Tuesday condemned Fornalik's charges to a fourth-placed finish in Group H.
"The Polish Football Association announces that on October 16, 2013 the coach Waldemar Fornalik ceased to be coach of the Polish national team," read a statement on the association's website, pzpn.pl.
"The president of the Football Association, Zbigniew Boniek, thanks Waldemar Fornalik for his one year's work with the national team."
Fornalik, who spent his entire playing career at Ruch Chorzow and managed the club for three years prior to succeeding Smuda, led Poland to just three wins from 10 qualifiers, drawing four times and losing a further three.
"It was a special time for me - great emotion, great commitment and work," Fornalik said. "Many thanks to all those with whom I have worked."