While it still remains to be seen whether the French star, 35, will play a romantic interest or a rival superhero to Hathaway's character of Selina Kyle – also known as Catwoman – the addition of Cotillard comes as a surprise, reports Monday's Le Parisien newspaper.
Sources also tell PEOPLE that the La Vie en Rose Oscar winner, who is expecting a baby in April with her French actor-director beau Guillaume Canet, signed onto the Batman project Saturday in Paris.
The Batman movie will also reunite Cotillard with her Inception director Christopher Nolan and costars Joseph Gordon-Levitt and Tom Hardy. Others in the cast include Michael Caine, Morgan Freeman and Gary Oldman.
According to sources, Cotillard will start shooting at London's Elstree Studios in mid-June, then move on to locations in Pittsburgh and Los Angeles this summer. Filming is expected to wrap in New York in November, with the picture's release set for July 2012.
Cotillard's next picture is Woody Allen's Midnight in Paris, which opens this year's Cannes Film Festival May 11 – with Cotillard expected on the red carpet.
By then, Bale might also be holding an Oscar of his own: he is nominated – and the frontrunner – as Supporting Actor this year, for The Fighter.