Celtic have signed midfielder Jiri Jarosik from Chelsea on a three-year deal for an undisclosed fee.
Celtic manager Gordon Strachan welcomes Jiri Jarosik
And the Scottish champions have the option of a further year for the 28-year-old Czech international.
"He has tremendous ability and experience of playing at the highest level," said manager Gordon Strachan.
"He is someone I feel will make an important contribution to the club as we look towards the challenges ahead, both at home and in Europe."
Jarosik spent last season on loan at Birmingham after failing to establish himself at Stamford Bridge.
He moved to Chelsea in January 2005 from CSKA Moscow, but made just 20 appearances, only six of them starts.
"It is a tremendous honour to sign for a club of Celtic's size and stature," said the player, capped 23 times for his country but not at the World Cup.
I am very excited about joining Celtic and hope to play a part in bringing even more success to the club
"It is a fantastic time to be joining Celtic, with the club again Scottish champions after doing so well last season and also with so much to look forward to in the Champions League.
"I am very excited about joining Celtic and hope to play a part in bringing even more success to the club."
Jarosik featured in 32 of Birmingham's games last season and was the club's joint top-scorer, with eight goals.
With Roy Keane retiring and the futures of Stilian Petrov and Neil Lennon unsure, Strachan needed to bolster his midfield and more signings are expected.
Club captain Lennon has yet to agree a new deal and Petrov has asked for a transfer.
Celtic chief executive Peter Lawwell added: "We are delighted to welcome Jiri to Celtic Park and are pleased to be able to bring a player of such quality to the club.
"We are pleased already to have signed a number of new players as Gordon looks to strengthen his squad.
"Supporters can rest assured that we will continue to support Gordon's plans for the future as we look ahead to our domestic and Champions League challenges."