Di Gregorio, of the Delko-Marseille Provence-KTM team, tested positive in a sample collected on 8 March for darbepoetin (dEPO).
The French rider finished 55th on stage 5 to Sisteron when the sample was taken and ended the race 22nd overall.
The UCI said the doping control was intelligence-led, planned and carried out by the Cycling Anti-Doping Foundation (CADF) in collaboration with the French anti-doping agency AFLD, and Central Office for Coordinating Environmental and Public Health Crime (OCLAESP).
In accordance with UCI anti-doping rules, Di Gregorio has been provisionally suspended until the adjudication of the affair, although the rider has the right to request and attend the analysis of the B sample.
Di Gregorio was kicked out of the 2012 Tour de France over suspicions of doping and later sacked by his then team, Cofidis, making his comeback a year later after having successfully sued the team over his dismissal.
He announced his retirement at the end of 2017 but decided to continue in 2018 with Delko-Marseille Provence-KTM.