Results of a second detection of the virus came on Friday, according to representatives from the Sewer Coronavirus Alert Network (SCAN).
The Oceanside Treatment Plant serves about a third of San Francisco's residents on the city's west side. The samples were taken Monday, June 20 and Thursday, June 23.
The samples "showed small concentrations of viral DNA for the disease," researchers said.
SCAN has been testing wastewater for COVID-19 during the pandemic. Earlier this month they began testing for monkeypox DNA.
"With two samples in the same area, we have confidence this is a true result," said SCAN researcher Alexandria Boehm, professor of civil and environmental engineering at Stanford University.
In a news release, SCAN said the relationship between levels of monkeypox DNA in wastewater and the number of cases in the community is not yet known. They will monitor to see if there are increasing or decreasing rates of detection.
As of this week, San Francisco has 10 confirmed and probable cases of monkeypox.
On Thursday Bay Area health officials urged awareness of the monkeypox virus as summer travel and gatherings get underway. That same day, Santa Clara County announced its first suspected case of the virus.
Monkeypox appear as distinctive rashes and sores that can look like blisters or pimples. It has seen outbreaks around the world in recent months and has disproportionately affected the some from the LGBTQ+ community, specifically men who have sex with men.
Health officials have said risk of monkeypox to the general public is currently low unless they engage in higher-risk behaviors, such as having sex with multiple sex partners, which can increase a person's risk of becoming infected.
There is treatment that can be used for monkeypox, but it’s generally used in severe cases.