The 52-year-old comedian is being brought in to replace 'The Hangover' star Galifianakis, who is unable to play the part due to scheduling conflicts, Deadline reports.
Carell, who starred in 'The 40-Year-Old Virgin' and 'Despicable Me', is one of the most popular comedians on the Hollywood scene and has been drafted in for the Peter Sollett-directed film, which is based on the 2007 documentary about Laurel Hester and Stacie Andree.
The script for 'Freeheld' centres on their fight to amend the Domestic Partnership Act, which affords pension benefits to domestic partners of all New Jersey public employees.
Carell is set to play the supporting role of Steven Goldstein, who advocates for Garden State Equality.
Last month, meanwhile, it was announced that Carell is to produce and appear in new Disney film 'Brooklyn Family Robinson'.
The comedian is to produce the movie with Charlie Hartsock and Vance DeGeneres, his partners at Carousel Productions.
'The Art Of The Deal' how you've never seen it before.
His wife is on the warpath now.