The district council-owned building has been upgraded for the first time in more than three decades.
It includes a new 33m pool, a shallow training pool, sauna and steam room.
The sports hall has been revamped and a £250,000 climbing wall has been built at the Bridgefoot centre.
The double Olympic gold medallist said: "It's an amazing venue for these guys so that they can learn a life skill as well as having fun.
"So many pools nowadays through this whole country have been knocked down and it's incredible that they've invested the money back to make a safe place."
It is the second time Adlington has visited the centre after she opened the temporary pool provided as part of the British Gas 'Pools for Schools Programme' earlier this year.
Other athletes who joined the celebrations included water polo player, Rosie Morris, who competed as the GB goalkeeper at the London Olympics in 2012.
Zoe Reeve, former member of the GB Synchro Squad and triple Commonwealth Gold Medallist and three-time Olympian James Goddard also attended the event.
Olympic swimming champion Rebecca Adlington has unveiled a £5m refurbishment of Stratford Leisure Centre.