The decision to veto the bill, which likely would have opened up the state to lawsuits from abortion rights supporters, comes at a time when Fallin is considered a possible running mate for presumptive GOP presidential nominee Donald Trump.
"The bill is so ambiguous and so vague that doctors cannot be certain what medical circumstances would be considered 'necessary to preserve the life of the mother,'" Fallin, a Republican, said in a statement.
The Oklahoma state legislature passed the legislation on Thursday. According to the bill's language, anyone who is found to have performed an abortion -- except in instances to save the life of the mother -- would be found guilty of a felony and could receive up to three years in prison.
"This bill is as direct an assault on Roe v. Wade -- and the Supreme Court's subsequent jurisprudence -- as anything we've seen before. If this law is upheld, then (the Roe decision) is meaningless," Steve Vladeck, a CNN contributor and law professor at the American University Washington College of Law, said Thursday.
Abortion rights groups quickly hailed Fallin's veto.
"This bill exposed the true motives behind restrictions across the country: to ban abortion," Dawn Laguens, the executive vice president of Planned Parenthood, said in a statement. "Governor Fallin would rather restrict abortion through back channels than do so outright. Fallin's policies are already punishing women: she has signed a forced delay for abortion, restrictions on medication abortion, and a Texas-style law designed to shut down health centers."
Nancy Northup, president and CEO of the Center for Reproductive Rights, said Fallin did "the right thing" but also called on other Oklahoma elected officials to "stop policing women's most personal, private decisions about their families, their lives, and their futures."
But Liberty Counsel, an evangelical group, called Fallin's veto a "despicable betrayal."
"As one who proclaims to be pro-life, her actions run counter to her words," Mat Staver, the group's founder and chairman, said in a statement. "This is a despicable betrayal of her word and of innocent children whose lives will be cut short because of her cowardly act. I encourage the Oklahoma legislators to veto the governor and Liberty Counsel stands ready to defend this bill."
Costs 'weighed heavily' on Fallin
Sources familiar with the governor's thinking told CNN that the decision to veto the bill "weighed heavily" on the anti-abortion rights governor, but that the "hundreds of thousands of dollars in legal fees" faced by the state from a near-certain Constitutional challenge to the bill eventually led to her veto.
Oklahoma is currently facing a budget crisis, something that spokesperson Michael McNutt told CNN the governor's office was focusing on instead of the growing buzz over the governor's chances to be on the ticket in November.
Trump fueled speculation about Fallin's prospects to be his running mate when he told Fox News' Greta Van Susteren that Fallin was "a fabulous person," but McNutt told CNN that Fallin hasn't "heard anything from the Trump campaign about being vetted." The governor told CNN in a statement earlier this month, however, that she is behind Trump "100%" and that "I would be very honored if I were to receive a call saying I need you to help make America great again."
Trump in the past has said women who get abortions if the practice should face "some form of punishment,"
though he later walked it back and said it was the doctor -- not the woman -- who should be punished if the procedure were to be outlawed.