The 6-3 ruling was a blow to the Republican National Committee, which sued to halt the change in the state's rules, which previously stated that all absentee ballots must be filled out in the presence of two witnesses or a notary public.
The state amended the rule to accommodate for the COVID-19 pandemic, which has some Americans -- particularly those who are medically vulnerable -- staying home and out of public spaces.
The Republican Party of Rhode Island said it was disappointed in the Supreme Court's ruling, which upheld lower courts' decisions that sided with the state's change.
"We fear that this decision will create more, not less confusion this election year. In Alabama, a state law requiring witnesses for mail ballots is constitutional, but in another, Rhode Island, it is unconstitutional," the party said. "Whether a state election law violates the U.S. Constitution now depends on the whims of your state's election officials. The authority of state legislators in establishing election laws has been undermined."
Rhode Island Secretary of State Nellie Gorbea praised the ruling, saying it allows voters to both maintain their health and more easily participate in the election process.
"Making it easier to vote safely from home by removing the burden of obtaining two witnesses or a notary is a common-sense step that will protect Rhode Islanders during this pandemic. We will mail out the requested mail ballots for our September 8 primary starting today," Gorbea said.