Klausmeier said she made the decision after talking to her family, friends and members of the clergy.
"It's an issue of fairness," said Klausmeier, who grew up Catholic but said the bill as written will not force ministers of any religion to perform same-sex marriages if they do not want to.
Klausmeier said she made her decision last week after speaking with a friend about the issue who "came in and said, 'My mother would really like it if I were able to get married.'"
"That really hit home for me," Klausmeier said. "I could be that mother."
Klausmeier is one of two state senators who represent the county who now say they will support a bill legalizing same-sex marriages in Maryland.
Sen. Edward Kasemeyer, who represents the 12th District that includes Catonsville and Arbutus, is also said to now support the bill.
Last week, Sen. Jim Brochin, a Towson Democrat, said of the same bill after years of opposing efforts to legalize same-sex marriage in Maryland.
Klausmeier and Brochin both represent districts that lean Democrat but tend to be conservative.
Klausmeier said she wasn't sure how her decision would be received in her district, which includes Perry Hall, Parkville and Overlea.
"I'm trying to keep as low a profile on this as I can," said Klausmeier. "This issue is pretty controversial. I think there would be blow back from either side no matter what I decided to do."