The Treviso prop, who has 17 caps in total, was reportedly in high demand but Sale have won the race to his signature, and Diamond believes De Marchi will be a major asset for the Sharks.
"Alberto's a very good player and we are absolutely delighted to have signed him," said Diamond.
"He's a good age, being just 25, and he's also versatile in that he can play both sides of the scrum at tight head and loosehead, though we see him as a tighthead in the main.
"And I think Alberto will fit in very well with our other props Vadim Cobilas, Ross Harrison and Fridge (Eifion Lewis-Roberts).
"Alberto is a good scrummager, he is very mobile around the pitch, he's also a good defender and we're very happy to have him on board.
"I've no doubt he'll prove to be a really big asset for us over the next few years."