The game launched on PC back in 2009, and recently hit the PlayStation 3 in both the US and Europe.
To celebrate, anybody that logs in between October 21st and October 31st will receive a free in-game jacket to commemorate the milestone.
"This benchmark is an honor and a true testament of SOE's success with Free-to-Play games," said SOE gaffer John Smedley.
"Free Realms delivers a truly captivating, quality gameplay experience that keeps our community engaged and continues to attract new players every day."
Sony Online Entertainment's also confirmed that a Hallowe'en mini event is set to take place within Free Realms between October 24th and November 7th. The event will feature various Hallowe'en themed mini-games and quests.