The 37-year-old country singer from Belton, Mo., beat runner-up Carly Rose Sonenclar, a 13-year-old schoolgirl from Westchester, N.Y., and teenage girl group Fifth Harmony on the finale that aired live Thursday night.
Stevens wins a $5 million recording contract.
More than 35 million votes were cast by viewers after Wednesday's performance show.
Besides Reid, judges this season included Demi Lovato, Britney Spears and series creator Simon Cowell.
Thursday's show was also the grand finale for Reid. Earlier this month, he said he wouldn't be returning to "The X Factor" next year. No replacement has been announced.