The 24-year-old has signed a three-year deal after heading for Wearside to hold talks. His arrival caps a good two days for the Black Cats, who earlier pushed through their move for South Korea striker Ji Dong-won after landing Connor Wickham on Wednesday.
Gardner, who cost his new club in the region of ?5million rising to ?5million, was City's top scorer last season and will be handed the task of providing goals from the middle of the park, where he is likely to take over from Jordan Henderson following his big-money switch to Liverpool.
'No-brainer': Craig Gardner has swapped the Championship for the Premier League
The Solihull-born player is a boyhood Birmingham fan despite beginning his senior career with derby rivals Aston Villa, and helped the Blues to a famous Carling Cup final victory over Arsenal last season.
However,the club's relegation from the Barclays Premier League made his departure almost inevitable, and the Black Cats were quick to make theirinterest known and then persuade Gardner that his future lay at the Stadium of Light.
Gardner told SAFC TV: 'It's unbelievable, to be honest. As soon as I found out Sunderland were interested, it was a no-brainer.
'Obviously, I have heard so much about Steve Bruce and his backroom staff and how good a chairman Niall Quinn is.
'It's such a massive club and their ambitions for this season meet my ambitions, and it was too good to turn down.'
The midfielder, who scored 10 goals in the last campaign, is confident he can bring the same potency to Wearside, where he is setting his sights high after hearing Bruce's recruitment plans, which have already landed him five new signings.
Coming through: Gardner has left Sunderland to join Birmingham
Hesaid: 'It means that the club wants to go places. Obviously, we want tofinish as high up the table as we can - we want to be in Europe next year and it's a great task to go and chase.
'It's a good goal for us and we can go and do it, and I am sure we will.'
Gardner, who scored 10 times for the Blues last season, revealed it did not take Bruce long to persuade him where his future lay after he headed for the north-east on Wednesday.
Hit man: Gardner will be ask to provide goals from midfield
He said: 'Speaking to him, he said, "Craig, once you get here, you will love it". I have been here 24 hours and I am so impressed with the place, the training ground, the staff.
'It's just an absolutely brilliant place to be, and hopefully we will settle down really quickly and I willstart hitting the back of the net.'
Gardner's arrival was announced just hours after Sunderland confirmed that Ji had completed his protracted move to the Stadium of Light, also for an undisclosed fee and on a three-year contract.
The 20-year-old former Chunnam Dragons frontman will join record signing Asamoah Gyan and fellow new boy Connor Wickham in a new-look frontline, and he too has high hopes ofmaking his mark.
And the 20-year-old is already looking forward to going head-to-head with compatriots Park Ji-sung and Lee Chung-yong when the Black Cats come up against Manchester United and Bolton respectively, although he will be doing them no favours.
He said: 'I am so glad to be able to meet other Korean players on such a big stage, but there is no such excuse for the opponent. I will do my best to defeat them.
'I am so satisfied to be able to come to a good club, a nice club in the world's number one league.
'I am happy because I have a chance to challenge the biggest players, and that gives me motivation.'
Bruce has made something of a reputation for himself by signing players from all corners of the world and Central and South America in particular.
Recent acquisitions Paulo da Silva, Cristian Riveros and Marcos Angeleri have struggled to make an impact on Wearside, and both the manager and his latest signing know it will take him time to adapt to a different lifestyle and a different type of football.
Ji said: 'It will not be easy. I am enjoying the life and studying English. I hope I could be a member of this team within six months.'
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