Camilla, 74, caught the virus for the first time just days after the Prince of Wales contracted the illness for a second time.
A Clarence House spokesman said: "Her Royal Highness The Duchess of Cornwall has tested positive for COVID-19 and is self-isolating.
"We continue to follow government guidelines."
Camilla is triple vaccinated, sources said.
Clarence House declined to confirm how Camilla was feeling or whether the prince and the duchess were isolating together on Valentine's Day.
The duchess carried out a string of engagements last Thursday - the day Charles tested positive.
Camilla described herself as "luckily" negative the same day, saying of her testing regime during a visit to Thames Valley Partnership in Buckinghamshire: "I've taken it so many times".
Concern for the Queen's health mounted after the monarch was in direct contact with eldest son Charles two days before he tested positive.
Buckingham Palace has continued to refuse to confirm whether the Queen had tested positive or negative, citing medical privacy, and saying last week only that she was not displaying any symptoms.
The 95-year-old spent time with Charles last Tuesday when the prince was carrying out an investiture on her behalf at her Windsor Castle home.
Camilla was mask-free when she met dozens of people on a visit to the Nourish Hub community kitchen in Notting Hill on February 10, where she joined a cooking demonstration.
The duchess helped one of the chefs add saffron oil to the pan of steamed rice, which was layered with green beans, tomatoes, quorn mince, cinnamon and turmeric to make an Iranian rice dish called Loobia Polo.
The team prepares up to 200 meals for diners visiting the hub each day.
Camilla did wear a mask for some of her visit to Paddington Haven, a sexual assault referral centre in west London, the same day.
According to government guidelines, when the prince tested positive, the duchess was not required to self isolate because she was fully vaccinated.
She has followed recommendations to begin taking lateral flow tests every day for seven days, in keeping with the rules.