It said his family was "shocked and devastated" by the discovery on Thursday of his body and that of a New Zealand woman on a beach in Mellitah, near the city of Sabratha, west of the capital.
The woman was his "close friend", the Foreign Office said in a statement, adding that its thoughts were with her family.
Libyan police Colonel Hassen Kamoka said a probe had been launched into the deaths, but that there still were no leads as to who the killers were or their motive.
De Salis had been working for First Engineering as a power manager, providing electricity generators to Tripoli, it said.
According to his LinkedIn profile, he began working for First Engineering as an account manager in April 2010, and before that was branch manager for OPS International Ltd in Tripoli for three years.
"Mark enjoyed his work in Tripoli and liked the Libyan people," the Foreign Office said.
"Mark enjoyed travelling and had travelled extensively. He was a decent and incredibly loyal man and he was loved by many. He will be sadly missed by his family and friends."
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