Croatia Under-21 defender Jozo Simunovic has joined Celtic from Dinamo Zagreb for an undisclosed fee.
The 21-year-old completed his move hours after fellow central defender Virgil van Dijk was sold to Southampton by Celtic for £13m.
Celtic manager Ronny Deila said: "I am sure he will be a great addition.
"He is a very talented young player and good on the ball, but he will also give us strength and stability in defence. I am sure he will be a success for us."
Simunovic, who has signed a five-year contract, was an unused substitute in both legs of Dinamo's Champions League play-off victory over Skenderbeu of Albania.
In the previous round, Simunovic played as Dinamo helped eliminate Norwegian side Molde after drawing 4-4 over two legs.
Molde are now in Celtic's group in the Europa League along with Ajax and Fenerbahce.
"I believe what we have done in the summer has given us a squad with real depth for the season ahead," Deila told Celtic's website.
"We already had a strong squad going into the summer period and we have now signed eight quality players in this transfer window.
"Again, we will once again look to strengthen in the January transfer window.
"We have the resources available to do this and this is something we always aim to do, ensuring that we are as well equipped as possible to bring our supporters as much success as we possibly can."
Simunovic was delighted to have completed his move, which came shortly after midfielder Ryan Christie agreed to join Celtic from Inverness Caledonian Thistle.
"I know we have a lot to play for this season and there are many exciting times ahead," he said. "This is my reason for joining Celtic.
"I want to win things in my career and I think I can do this at Celtic."