Edmunds estimated the three Detroit-based automakers spent a combined $2.6 billion on sales incentives in the past month, while Japanese automakers spent $823 million.
All of the six leading automakers offered bigger discounts in June than they had in May with the exception of General Motors , Edmunds said.
In a significant departure from usual practice, the three leading Japanese automakers all stepped up their discounting in the face of slack demand, Edmunds said.
"The competitiveness of the marketplace seems to be catching up with the Japanese heavyweights,'' Edmunds analyst Jesse Toprak said in a statement.
Toyota Motor announced in mid-June that it had begun offering rebates of up to $3,500 or interest-free loans for five years on its all-new Tundra pickup truck.
Edmunds estimated that Toyota's average incentive was $1,308 per vehicle in June, up from $1,128 in May.
Chrysler, which has been offering deals for months to support sales of its aging minivan and truck line-ups, had the industry's highest incentives at $3,962 per vehicle in June, up from $3,831 in May, Edmunds said.
Auto sales incentives are widely tracked by analysts as an indication of the relative profitability of competing automakers and the pressure that they face to move inventory.
Automakers do not typically disclose how much they spend on incentives, which can include concessional financing, cash rebates or additional payments to dealers.
Both Ford Motor and GM rolled out end-June summer sales campaigns featuring interest-free financing on a range of vehicles aimed at boosting sales results.
Edmunds estimated that Ford's average incentive spending rose to $3,187 in June from $2,942 in May.
Both GM and Ford headed into the year with a strategy of throttling back on incentive spending as part of a strategy to move away from the kind of blowout sales and volatile results that dogged their results earlier this decade.
Rounding out the six major automakers, Edmunds estimated that Honda Motorhad spent an average of $1,397 on incentives in June, from $1,300 in May.
It estimated average incentive spending for Nissan Motor at $2,218, from $1,943 in May.
By segment, the biggest deals were on minivans, which carried an average discount of $3,900, and full-size trucks, which had an average incentive of $3,864, Edmunds said.