The captain of the national swimming team raced a pair of dolphins - King, 19 years old, and Leah, nine-years old - at a specially arranged event in a swimming pool in Torvaianica, about 30 miles (50km) south of Rome.
The dolphins had to swim about twice as many lengths as Magnini, but they overtook him to win in the last few yards (meters).
"I never had such tough opponents," joked the two-time world champion. But he said he "fell a bit in love with dolphin Leah" and hoped to see her again.
Magnini - nicknamed Superpippo - won gold in the 100 meter freestyle at the 2005 world swimming championships in Montreal and at the 2007 event in Melbourne.