The San Francisco-based tech giant also paused "future office reopenings, effective immediately," a Twitter spokesperson told FOX Business in a statement.
"After careful consideration of the CDC's updated guidelines, and in light of current conditions, Twitter has made the decision to close our opened offices," the spokesperson said." We’re continuing to closely monitor local conditions and make necessary changes that prioritize the health and safety of our Tweeps."
The move comes just after the company reopened both offices on July 12, even though it is allowing some employees to work from home indefinitely.
In May 2020, the company told employees that "opening offices will be our decision, when and if our employees come back, will be theirs."
It was part of an effort to provide employees with the flexibility to determine how and where they work best, according to Twitter.
Now, the company is tightening its in-person protocols as the delta variant that continues to fuel infections surges.
Twitter officially announced the closure of both offices just one day after the Centers for Disease Control and Prevention (CDC) recommended that even vaccinated people return to wearing masks indoors in parts of the U.S.
"The more we learn about this virus and the delta variation, the more we have to be worried and concerned," President Biden said.
The Associated Press contributed to this report.