The Red Bull KTM Ajo rider is self-isolating and will miss this weekend's Misano round of the second-tier championship, in line with the protocols set by the race organisers.
It represents a blow to Austrian GP race winner Martin's title ambitions as he was only eight points behind Championship leader Luca Marini of Sky Racing Team VR46.
There has also been one positive result returned for a mechanic from a different team.
"They have not entered the paddock for the event and will not be able to attend the San Marino Grand Prix," MotoGP said in a statement.
MotoGP have conducted more than 14,000 tests for the novel coronavirus since before the season's resumption in July and has detected and isolated each positive case diagnosed. (Reporting by Hardik Vyas in Bengaluru Editing by Christian Radnedge)