Joss Whedon's followup to his 2012 superhero ensemble picture is due out in theaters May 1, 2015. It will once again star Downey Jr. as Iron Man, Mark Ruffalo as the Hulk and Chris Hemsworth as Thor.
Marvel announced on its website Thursday Spader will play the main baddie Ultron.
"James Spader will face off against Earth's mightiest heroes as the villain in Marvel's 'Avengers: Age of Ultron,' the sequel to 2012's record-breaking 'Marvel's the Avengers.' The Emmy Award-winning actor will play Ultron in the Avengers' much anticipated return to the big screen from writer-director Joss Whedon," a message on the website said.