Revenue reached $12.6m (£10.3m/€12.0m), more than double $6.1m in May of last year and also marginally higher than $12.4m in April of this year.
Of this total, $11.6m was attributed to online sports betting activity, while the remaining $1.0m came from retail sportsbooks in Iowa.
In terms of handle, players wagered a total of $147.9m in sports during the month, which was 28.7% higher than $114.9m in May 2021, but 16.6% lower than $177.4m in April this year.
Consumers staked $134.3m online in May, in addition to $13.6m at retail locations, winning a total of $135.3m across the two channels in the process.
Diamond Jo in Dubuque and its FanDuel sportsbook retained top spot in the market with ease, posting $3.8m in revenue off a $31.6m handle.
DraftKings partner Diamond Jo in Worth placed second with $1.8m and a $20.2m handle, then Wild Rose in Jefferson, also a DraftKings partner, with $1.4m in revenue from $18.8m in player bets.
The Iowa Racing and Gaming Commission also revealed that regulated sports betting activity in May generated $850,598 in tax revenue for the state.