The tweet was in response to the former California governor's acknowledgement that he fathered a child with a member of his household staff.
"Another guy guv admits 2 cheating on his wife. Maybe we need more women governors. Guys: keep ur pants zipped, for Pete's sake," Granholm wrote in the tweet, which was labeled "(hash)Arnold."
Former press secretary Liz Boyd confirmed to The Associated Press that Granholm typed the tweet herself.
The revelation of Schwarzenegger's child apparently prompted wife Maria Shriver to leave the couple's home before they announced their separation last week.
"There are no excuses and I take full responsibility for the hurt I have caused. I have apologized to Maria, my children and my family. I am truly sorry," Schwarzenegger told The Los Angeles Times in a statement that also was sent to the AP on Tuesday.
The child was born before Schwarzenegger began his seven-year stint in public office. Granholm, a Democrat, and the GOP's Schwarzenegger have crossed paths before.
Both were the subject of presidential speculation a number of years ago, although they are ineligible to run.
Despite becoming U.S. citizens, Schwarzenegger, from Austria, and Granholm, from Canada, are prohibited from seeking the presidency because such a campaign would violate the constitutional requirement that presidential aspirants be natural-born citizens.
And in 2009, Granholm got in a few partisan digs at Schwarzenegger during the Gridiron Club's annual political roast in Washington. She was chosen to give the Democratic remarks and Schwarzenegger spoke for Republicans at the dinner.
Granholm, Michigan governor from 2003 through last year, now works with the Pew Environmental Group on its efforts to promote clean energy policies and is a regular contributor to NBC's "Meet the Press." She recently was elected to the Dow Chemical Co. board of directors.