The bill, approved by the Midwestern state's Republican-controlled legislature just two days earlier, was expected to trigger a legal battle, which conservatives hope will land the flashpoint social issue back at the US Supreme Court.
"I understand and anticipate that this will likely be challenged in court," Republican Governor Kim Reynolds said in a statement after the signing.
"However, this is bigger than just a law. This is about life. I am not going to back down from who I am or what I believe in," she said.
Considered the most restrictive in the country, critics say the so-called "fetal heartbeat" law will deny abortions to many women before they even know they are pregnant -- with an exception for victims of rape or incest.
The state legislature approved the bill Wednesday after heated debate.
"We are taking a courageous step... to tell the nation that Iowa will defend its most vulnerable," Republican state legislator Shannon Lundgren said on the House floor.
Beth Wessel-Koreschell, a Democratic legislator, countered that lawmakers believe "they know more than the medical experts on what is good medical practice."
'Clearly unconstitutional'
Abortion foes hailed the law as the first step in potentially dismantling Roe v. Wade -- the landmark 1973 Supreme Court decision that granted women the legal right to an abortion.
But the Supreme Court has previously been hostile to "heartbeat" laws. In 2016, the high court rejected appeals by both North Dakota and Arkansas to preserve similar abortion restrictions, which had been struck down by lower courts.
The law's supporters in Iowa argued that the Republican administration of President Donald Trump may have an opportunity to appoint more conservatives to the high court in the next few years, just in time for this latest law to wind through the legal system.
As if on cue, the American Civil Liberties Union on Friday announced plans to file a lawsuit.
"The ACLU of Iowa, along with Planned Parenthood, will not let this stand and will file a lawsuit to block this clearly unconstitutional law," the civil rights group said on Twitter.
The debate over abortion rights continues to rage across the country, especially in areas where conservatives are in power.
The Guttmacher Institute, a pro-abortion research group, has tallied 400 new abortion restrictions in states between 2011 and 2017.
Consequently, there have been numerous legal challenges.
The previously most restrictive abortion ban was enacted in March in the Southern state of Mississippi. It outlawed the procedure after 15 weeks of pregnancy.
A judge halted the law while a lawsuit moves forward.
A similar fate befell Kentucky's new ban on second-trimester abortions. It was halted by a federal court in April.
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