He says, "We are Americans, and our country is under threat."
The Democrat said Monday the act could slow the death toll in the COVID-19 pandemic, which he noted could climb by 200,000 more before a vaccine is widely available.
Biden said, "Please, I implore you, wear a mask." He noted masks could save the lives of older people, children and teachers and added: "It could even save your own life."
SEE ALSO: 2 doctors with Delaware Valley ties tapped for Biden's COVID-19 Advisory Board
Biden notes that he doesn't take office until Jan. 20 but is assuming a public leadership role in the fight against the pandemic ahead of being sworn in.
"I'm pleased President-elect Biden is talking about wearing masks. It's a simple thing to do," said Philadelphia Health Commissioner Dr. Thomas Farley.
COVID-19 cases have been climbing this fall. Philadelphia is averaging 600-700 cases a day, over the last several days. The city is now advising those who participated in election protests and celebrations to quarantine.
"We're seeing a spike right now, even without the election week changes. We don't want it to be any worse than that," he said.
WATCH: Biden cheers COVID-19 vaccine progress but says masks remain a must
The Pennsylvania Department of Health announced 3,402 new COVID-19 cases Monday, in addition to 2,909 new cases reported Sunday, bringing the statewide total to 234,296.
Residents are anxious what a climb in cases would mean for their families. Others are skeptical of the medical recommendations to wear masks.
No new restrictions have been announced in Pennsylvania, but in New Jersey, Governor Phil Murphy has announced a series of new measures to combat the rise in cases.
Murphy said bars and restaurants must halt indoor dining from 10 p.m. to 5 a.m. beginning Thursday in an attempt to control a resurgence of the coronavirus.
SEE ALSO: Gov. Murphy halts indoor dining from 10 p.m. to 5 a.m. across New Jersey
"We need to change our mindset, we have to shake off the pandemic fatigue that we all feel," Murphy said during a news conference in Trenton.
Takeout and outdoor dining may continue past 10 p.m., Murphy, a Democrat, said. Another change allows restaurants to seat tables closer than 6 feet (2 meters) apart if they erect a barrier. He also said that all indoor interstate athletic events will be prohibited.
Over the past seven days, an average of about 2,000 people per day have been infected by the virus in the Garden State, up from a rate of about 370 per day in August.
-The Associated Press contributed to this report.