Green Day singer rehab
A video clip of the outburst shows Armstrong suddenly stop performing mid-song when he is told Green Day have one minute left on stage.
He then launched into an expletive-laced tirade, saying he thought the group's set had been cut short by 20 minutes with rumours that Usher was to be given more time.
The California-based band later apologised to the festival and promoter Clear Channel in a statement on their website.
"Billie Joe is seeking treatment for substance abuse," it said.
"We would like everyone to know that our set was not cut short by Clear Channel and to apologise to those we offended at the iHeartRadio Festival in Las Vegas.
"We regretfully must postpone some of our upcoming promotional appearances."
Earlier this month, Green Day were forced to cancel a concert in Italy after Armstrong was treated in hospital.
His bandmates said he had "severe dehydration", though he recovered in time to perform at the 2012 MTV Video Music Awards.
It is not known whether Armstrong's treatment will have an impact on Green Day's US tour, which is due to start in November.
The group, who have sold more than 65 million records worldwide and won five Grammys since forming in the late 1980s, are preparing to release their new album Uno.