He took a regularly-scheduled COVID-19 rapid self-test and it showed a positive diagnosis, which was later confirmed with a second rapid self-test.
Lamont is currently awaiting the results of a PCR test.
The governor's office says it has also conducted contact tracing to identify and inform close contacts.
Lamont, 68, received his second booster shot on March 31.
"He is grateful that he is receiving protection from the second booster shot he received last week," his office wrote in a statement. "The governor continues to encourage everyone to get vaccinated and receive their boosters in order to protect themselves and their loved ones."
Lamont, a Democrat, is described as feeling good and not experiencing any symptoms.
Per CDC guidance and out of an abundance of caution, the governor will quarantine at home for the next five days and not attend any in-person events or meetings.
Lamont will continue to monitor himself for symptoms and will take another PCR test in several days.
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