On Wednesday, the total of confirmed cases in the US reached 6,600, mostly among men who have sex with men.
It is hoped the declaration will improve the availability of data on infection rates, said the US Centers for Disease Control and Prevention Director Rochelle Walensky.
The POTUS wrote on Twitter: “I remain committed to our monkeypox response: ramping-up vaccine distribution, expanding testing, and educating at-risk communities.
“That’s why today’s public health emergency declaration on the virus is critical to confronting this outbreak with the urgency it warrants.”
Health and Human Services Secretary Xavier Becerra said at a briefing: “We’re prepared to take our response to the next level in addressing this virus, and we urge every American to take monkeypox seriously.”
The United States now has the most cases in the world, having spread there from Europe.
The Biden administration has been criticised for its handling of the outbreak, with vaccines in short supply and sexual health clinics heavily underfunded, reported Reuters.
Last month the World Health Organisation declared monkeypox a “public health emergency of international concern”, its highest alert level.
Many of the vaccines and treatments being employed were historically used to treat smallpox, a disease which has now been eradicated.
The virus was first identified in monkeys in 1958.
Symptoms of the disease include fever, aches and pus-filled skin lesions, although people tend to recover from it within two to four weeks.
It is spread through close physical contact and is rarely fatal in people.
Chief medical adviser Anthony Fauci said on Thursday that it was critical to engage leaders from the gay community as part of efforts to control the outbreak, but was cautious to avoid stigmatising members of that community.
He said: “Engagement of the community has always proven to be successful.”