* Kovac is a former Bayern player
* Croat had successful spell at Frankfurt (Updates with Kovac, Eintracht reactions)
BERLIN, April 13 (Reuters) - Croatian Niko Kovac will succeed Jupp Heynckes as Bayern Munich coach next season after agreeing a three-year contract, the German champions said on Friday.
Treble-chasing Bayern had tried to retain Heynckes, who had come out of retirement to take over in October until the end of the season, but the 72-year-old turned down an extension of his current deal.
"Niko Kovac will be the new coach from July 1, 2018," said Bayern sports director Hasan Salihamidzic in a statement. "We agreed yesterday on a three-year contract.
"Niko was a Bayern player, he knows the people in charge as well as the structure and the club's DNA. We are convinced he is the right coach for the future of Bayern."
The 46-year-old, who had a long playing career in the Bundesliga, was at Bayern from 2001-03. He joins from Eintracht Frankfurt, where he had a contract to 2019, for a reported transfer fee of just over two million euros ($2.5 million).
After the end of his playing career Kovac was briefly an assistant coach in Austria.
He then took over Croatia in 2013 for two years before signing with relegation-threatened Eintracht Frankfurt in 2016 and turning them into Champions League contenders this season.
The decision to appoint Kovac marks a change of approach for success-driven Bayern, who had previously opted for big-name coaches. Recent appointments have included Champions League winners Heynckes, Louis van Gaal and Pep Guardiola.
Just like when Guardiola succeeded Heynckes in 2013, Kovac could potentially start work after a treble-winning campaign, with Bayern having already secured the Bundesliga title, their sixth consecutive league crown.
They are also through to the Champions League last four where they will face Real Madrid and take on Bayer Leverkusen in the German Cup semi-final next week.
"I received a call (from Bayern) yesterday and a contract offer," Kovac told reporters. "I accepted it yesterday. I informed the team this morning. But one thing is clear and that is that it's the team that matters not me. It is what happens with Eintracht in the coming weeks."
Eintracht are in fifth place with five games left, two points off fourth-placed Leverkusen. The top four teams qualify for next season's Champions League group stage.
Kovac's brother Robert, also a former Bayern player, will join him from Frankfurt as his assistant.
Frankfurt sports director Fredi Bobic said the club had been surprised by the quick decision.
"We were obviously surprised especially in this phase (of the season) we are in which is extremely important for us," Bobic said.
"We built up something quickly and we now want to reap the fruits of our efforts. Obviously the timing of this announcement is inconvenient." ($1 = 0.8122 euros) (Reporting by Karolos Grohmann Editing by Alison Williams/Christian Radnedge/Toby Davis)