"I am not going to back down from who I am or what I believe in", she said.
The bill puts Iowa, a mostly rural state in the midwest that famously swings in elections, in the forefront of restricting abortions.
"I understand and anticipate that this will likely be challenged in court", the governor said in a statement. "This is about life".
According to CNN, Reynolds argued that it was "immoral to stop an innocent beating heart.my faith leads me to protect every Iowan, no matter how small".
"Today's actions to ban abortion are an embarrassment to Iowa and they will remain a blemish on our state for the foreseeable future", advocacy group Planned Parenthood Voters of Iowa said in a statement.
In Iowa, the same Republican-majority Legislature passed a 20-week abortion ban past year.
In the legislature, not a single Democrats voted for the bill and six Republicans also voted against it, according to the Associated Press.
The so-called "Heartbeat Bill" prohibits abortions in Iowa when a fetal heartbeat is detected, which is about six weeks into pregnancy.
"It is clearly unconstitutional and it effectively blocks the right (to) an abortion for most women", she said.
The governor of the USA state of Iowa signed into law Friday a ban on abortions once a fetal heartbeat is detected, which occurs as early as six weeks into a pregnancy.
Reynolds issued a statement Friday saying "this is about life".
Meanwhile, organizations that oppose abortion rights cheered the governor's decision. "She is following through on her pledge to the people of Iowa that she is 100 percent pro-life". "While Roe has not yet been reversed, it has been soundly rejected in the court of public opinion".
As writer Maureen Shaw noted, Iowa likely passed the legislation as an attempt to overturn Roe v. Wade, the landmark Supreme Court decision that prevented states from outlawing abortion during certain points in a pregnancy. "There's nothing hidden here about the agenda".