The officer has been moved into isolation at a different location after giving a positive reading on a rapid-turnaround lateral flow test.
A total of 12 officers who have come in close contact or are in the bubble of the one who tested positive are also self-isolating.
It was not immediately clear whether any of those involved would have come into close proximity of the G7 leaders, including Boris Johnson and Joe Biden, who are meeting at the Carbis Bay beach resort in Cornwall.
The scare comes after a St Ives hotel hosting media and security personnel for the summit closed temporarily for deep cleaning after members of staff tested positive.
The officers involved are taking PCR tests to confirm whether the initial positive test was accurate and whether the virus has spread more widely among them.
In a statement, Devon and Cornwall Police said: “As part of our testing regime, during the early hours of 11 June we have identified one officer who is currently supporting G7 policing and accommodated on the ferry, has given a positive lateral flow test for COVID-19.
“The officer, plus those who have come into close contact, are currently self-isolating at another designated location.
“All who have come into close contact or are in the bubble of those who tested positive are also currently self-isolating which equates to 12 officers in total. The next stage is for those who have tested positive to undertake a PCR test.
“We are continuing to follow the advice of Public Health England.”
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