Voters in Maine, Maryland and Washington state have approved same-sex marriage in local referendums on social issues.
It is the first time gay marriage has been backed by popular vote despite 35 such polls elsewhere in the US.
Gay marriage is permitted in six states, but those laws were passed by legislators or by courts.
In other referendums, voters in Colorado and Washington voted to legalise recreational use of marijuana.
In California, an attempt to abolish the death penalty was defeated. Meanwhile, Massachusetts voters struck down a measure that would have legalised physician-assisted suicide for terminally ill patients.
Gay marriage laws
The victory of proponents of gay marriage in three states surprised analysts.
Maine became the first state to pass a law in favour of the measure by a popular vote.
Gay marriage referendums were also held in the states of Maryland and Washington, where opponents of same-sex marriage were seeking to reverse legislation already on the statute books. Those attempts were unsuccessful.
Brian Ellner, the head of the pro-gay marriage group, The Four, said: "It's enormous. We have truly made history."
"Having the first states approve marriage by a popular vote changes the narrative and sends an important message to the Supreme Court."
In a fourth state, Minnesota, the electorate rejected an attempt to define marriage in the state constitution as "between one man and one woman".
Voters in 38 states have been asked to decide on 176 ballot initiatives, ranging from highly contentious to simply unusual.
Colorado and Washington appear to be heading towards a conflict with federal authorities after voters approved measures allowing the possession and sale of marijuana for recreational use.
Colorado's Democratic Governor John Hickenlooper said: "Federal law still says marijuana is an illegal drug, so don't break out the Cheetos or gold fish too quickly."
The US Department of Justice said in reaction to the vote that its drug enforcement measures would remain unchanged. Marijuana is classified as an illegal narcotic.
In Colorado, the measure would allow people over the age of 21 to possess up to an ounce (28g) of the drug, and to cultivate up to six marijuana plants.
Separately, measures to approve the medicinal use of marijuana were on the ballot in three states, including Massachusetts, where it has been passed.
Other measures included the clear labelling of genetically-modified foods in shops in California, and the use of condoms by porn stars in Hollywood.
The state of Oregon has held perhaps the strangest vote, on the use of improved grammar and spelling in the state's constitution.