The justices said in a 6-3 ruling Thursday that people can receive the subsidies even if they don't buy their health plans on a state-run exchange.
Arizona is one of 34 states that don't run their own marketplace for health insurance.
Gov. Doug Ducey signed a law earlier this year that barred the state from setting up its own exchange. The high court decision means he won't have to come up with another plan to keep people receiving subsidies insured.
The Supreme Court is considering two other cases of major importance to Arizona: The constitutionality of the state's congressional district maps and whether same-sex marriage should be legal nationwide.
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