Moore, 53, is set to play Page's love interest in the Peter Sollett-directed drama about a couple battling to secure pension benefits after one of them is diagnosed with a terminal illness.
The film's cast was announced a day after Page gave her famous coming out speech at the Human Rights Campaign's first annual THRIVE conference in mid-February.
"It was really wonderful," Moore told E! News of Page's speech. "I was very happy for her."
The Oscar-nominated actress also said she was "excited" about her upcoming collaboration with the Juno star.
Freeheld is currently in the pre-production stage.