The initiative, known as Amendment 64, makes Colorado the second state to put a measure to allow recreational marijuana on the Nov. 6 ballot. Washington state put similar measure on the ballot last month.
Legalizing marijuana for recreational purposes was defeated by 54 percent of California voters in 2010.
Mason Tvert, co-director of the Campaign to Regulate Marijuana Like Alcohol, one of the measure's chief proponents, said the campaign would use the next eight months to build a "broad base of support" across the state.
"Coloradans have a chance to make history this November, and we believe they are ready to do just that," Tvert said in a statement.
Proponents initially came up short of the needed 86,105 valid signatures, but given a chance to collect more signatures, they handed in another 14,000, The Denver Post reported.
Nearly 7,000 of those were found valid, giving proponents about 90,500, the Secretary of State's office said.
The measure would legalize possession of as much as 1 ounce of marijuana for adults age 21 and older. It would also let people grow as many as six marijuana plants in their home.
Specially regulated stores would be permitted to sell marijuana, but communities would have a right to ban such businesses.
State lawmakers would create a special marijuana tax, with the money going to education.