The Nerazzurri shot-stopper has no serious symptoms, but his recovery from the illness is taking longer than usual.
La Gazzetta dello Sport reports this is the third time Sportiello has been tested for coronavirus. The first test dates back to March 20, which came back positive.
A second swab was taken and came back negative, but a third test shows that the goalkeeper remains positive.
As a result Sportiello will continue to remain in quarantine in his home in Urgnano.
The 27-year-old is the only Atalanta player to test positive for the virus, even though the club find themselves in one of the hardest hit regions in Italy.
Sportiello has featured in four matches for the Nerazzurri this season after returning from a season on loan at Frosinone.