In a message on Instagram, the Italian said that he was diving back into the pool “for me, for my family, for my country”.
Contacted by AFP, the Italian swimming federation did not comment on the announcement.
Magnini took gold in the 100 metres freestyle at the 2005 and 2007 world championships, and picked up bronze for Italy in the 4x200m freestyle relay at the 2004 Olympics.
In February, the Court of Arbitration for Sport overturned a four-year doping ban handed to him by Italy’s anti-doping agency Nado in November 2018.
Magnini had been banned following criminal proceedings against a rogue nutritionist, Guido Porcellini, who was banned for 30 years by the Italian authorities for trafficking doping products and counterfeit medicine.
Magnini’s name was found among the evidence, which revealed certain products to be destined for the swimmer, although investigators conceded he may not have used or even received them.
“This was not an easy choice, but I decided to come back,” added Magnini, who retired in December 2017. “I almost hit rock bottom but rather than knock me out, all this gave me the motivation to fight, overcome and start competing again.”
He was also famous in Italy for his long-term relationship with former girlfriend and 2008 Olympic champion Federica Pellegrini, one of the country’s biggest sport stars.