"His Holiness sends condolences to all who mourn his loss, and in a special way to the members of Ambassador Karlov's family," read the telegram sent to Russian President Vladimir Putin on the pope's behalf.
"Pope Francis assures you and all the people of the Russian Federation of his prayers and spiritual solidarity at this time," the telegram concluded.
Karlov was shot dead by 22-year-old Mevlut Mert Altintas, an off-duty policeman, while he was speaking at a photographic exhibition in the capital.
Altintas was subsequently gunned down by Turkish special forces.