Ms Reynolds, a Republican, signed the “Back the Blue” bill into law on Thursday.
Under the new law, rioting becomes a felony, rather than a misdemeanour, while penalties for blocking streets will also be raised.
“We encourage First Amendment rights to protest peacefully, but if you break the law, you’re going to be held accountable,” the Iowa governor said, according to KCCI-TV in Des Moines.
Ms Reynolds said anyone caught rioting, looting or attacking police officers would be “punished to the full extent of the law.”
“The public peace is too important, and the safety of our officers too precious, to tolerate destructive behaviour,” the governor said.
The new law comes in the wake of widespread protests across the country following the May 2020 death of George Floyd, a Black man who was killed after a police officer, Derek Chauvin, who is white, kneeled on his neck for more than nine minutes in an arrest over the alleged use of a counterfeit $20 note.
Speaking at a press conference on Thursday morning, Democratic members of the Iowa Legislative Black Caucus said Ms Reynolds had not reached out to consult them when putting together the legislation, according to The Des Moines Register. They also accused her of abandoning the push for a ban on racial profiling by police officers.
“Instead of furthering an important discussion about anti-racial profiling measures and modernising our public safety departments, Gov Reynolds took a giant step backwards,” Iowa Democratic Party Chair Ross Wilburn, a representative from Ames, said.
With Ms Reynolds having signed a bill into law last year banning chokeholds and seeking to address police misconduct, the governor said: “There’s no contradiction between steadfast support for honourable and selfless police officers — the vast majority — and a commitment to improving policing,” according to the Register.
“There’s no contradiction between world-class investigation and treating victims of crime the way we ourselves would want to be treated. And there’s no contradiction between vigorous policing and the community outreach that builds trust between law enforcement and everyday Iowans,” she said.
In a statement posted on Twitter, the American Civil Liberties Union’s Iowa branch also took aim at the law, however, slamming it as an “effort to shut down well-founded public criticism of abuses by law enforcement and government, especially from Black Lives Matter activists”.
“Because this law aims to stifle lawful protesters, it is nothing less than an attack on free speech in our state,” the ACLU body said.
The law, the ACLU of Iowa said “takes our state in the wrong direction”.
The Independent has contacted Ms Reynolds’ office for further comment.