John (or Giovanni) Visconti (died 1275) was the Judge of Gallura from 1238 to his death.
However, Enzo, the husband of his Ubaldo's widow Adelasia of Torres, seems to have taken control of both Gallura and Logudoro and was granted the title King of Sardinia by his father, the Emperor Frederick II.
John annexed a third of it — Ogliastra, Quirra, Sarrabus, and Colostrai — to Gallura.
John subsequently remained mostly on the Italian peninsula, participating in the wars between the Guelphs and the Ghibellines on the side of Pisa.
He was later re-married to a daughter of Ugolino della Gherardesca, Count of Donoratico.