The Hawkeye State registered handle of $8.6m in its first 17 days of sports betting operations.
Iowa was the first state to launch online sports betting on the same day as retail but, to wager online, bettors must first register in-person at one of the state's casinos.
Online sports betting made revenue of $424,129 for August, with William Hill and Elite Sportsbook powering operations for seven of the 19 casinos.
DraftKings and FanDuel are both licensed in the state, with their launches pending.
FanDuel partnered with Boyd Gaming to offer sports betting at two Diamond Jo Casinos in the state, while DraftKings signed with casino operator Wild Rose.
Prairie Meadows Racetrack and Casino was the top taker in Iowa for both retail and online, registering revenue of $295,748 and $237,023, respectively.
Ameristar II, which does not currently offer online sports betting, made the second-highest total for August, $480,650.
Taxes of $145,915 were paid in August, based on Iowa's rate of 6.75%, one of the lowest in US sports wagering.
