The news comes in the wake of Armstrong's widely publicized, guitar-smashing tantrum at the iHeartRadio Music Festival in Las Vegas on Friday, during which Armstrong delivered a expletive-fueled rant about the band's performance time being cut short.
"One minute left, one minute (expletive) left. You're gonna give me (expletive) one minute? ... I'm not (expletive) Justin Bieber, you (expletive)," he said.
Afterward, the band released a statement apologizing.
"Billie Joe is seeking
It was not known where Armstrong is getting rehab.
Armstrong was hospitalized in early September in Italy, for what was rumored to be both dehydration and stomach problems, but the band has made several appearances since then.
However, in the wake of the Vegas tirade and announcement of Armstrong's entrance into rehab, Green Day has postponed its Friday show at the Fillmore in San Francisco, concert promoters said Monday.
No makeup date was available, but organizers said additional information would be coming soon. Concert promoter Live Nation said tickets purchased for the Friday show would be honored on the new date.
On Green Day's website, the band's upcoming tour of intimate venues is listed as still on schedule. That tour kicks off Oct. 27 in New Orleans and includes a Dec. 1 show at Memorial Auditorium in Sacramento, a Dec. 2 show at Grand Sierra Resort in Reno and a Dec. 4 stop at the Santa Cruz Auditorium.
Green Day released the album "¡Uno!" today. It is the first in a trilogy of albums; the second, "¡Dos!" is to be out in November, with "¡Tré!" slated for release in January.
The Associated Press contributed to this report.