The 39-year-old will play a rival anchor and nemesis to Will Ferrell's Ron Burgundy in the comedy sequel, according to Deadline.
Marsden is best known for playing Cyclops in the X-Men film series, and has also starred in Enchanted, Superman Returns and 27 Dresses.
Bridesmaids actress Kristen Wiig was recently confirmed to be joining Anchorman 2 alongside returning stars such as Paul Rudd, Steve Carell and David Koechner.
Terence Stamp was also reported to be wanted for a "Rupert Murdoch-type" role in the Adam McKay-directed follow-up.
Anchorman: The Legend Continues is due for release on December 20.
Watch Paul Rudd and Judd Apatow discussing Anchorman 2 with Digital Spy below: