The ranking is based on sales on the NFL's official online store from May 1-May 31.
"It's surreal for me, it's such an honor," Mariota said Wednesday in a radio interview with The Midday 180 in Nashville. "For me it's one of those deals, looking back on it in the future, it's a crazy, crazy deal for sure."
New England Patriots quarterback Tom Brady was third on the list. That suggests strong support for Brady over the past month, though Brady has worn the same jersey for the past 15 seasons, longer than any other player whose jersey finished in the top 15 best-selling jerseys in May.
Rounding out to the top 10 were Seattle Seahawks quarterback Russell Wilson, New York Giants wide receiver Odell Beckham Jr., New England Patriots tight end Rob Gronkowski, San Francisco 49ers quarterback Colin Kaepernick, Dallas Cowboys wide receiver Dez Bryant, Green Bay Packers quarterback Aaron Rodgers and Denver Broncos quarterback Peyton Manning.
The Super Bowl champion Patriots finished the month as the best-selling team, followed by the 49ers and Seahawks.
ESPN Titans reporter Paul Kuharsky contributed to this report.