Berbatov's agent, Emil Danchev, had made it clear earlier in the window that his client was unhappy at Fulham and was seeking a move, with reports indicating that the Bulgarian was set for a return to former club Tottenham.
However, Monaco stepped in and had a bid accepted by Fulham for the former Manchester United player, who is seen by the Ligue 1 side as short term cover up front following Falcao's cruciate ligament injury which has ruled the Colombian out for the rest of the season.
The 33-year-old Berbatov flew to Monaco for a medical on Friday afternoon, with the deal confirmed on Friday evening - though there is still some mystery, with Monaco insisting they have only loaned the player and Fulham announcing that the Bulgarian has left Craven Cottage for good.
Also on Friday night Philippe Senderos made a surprise permanent move to Valencia. The Swiss defender joined for an undisclosed fee after three-and-a-half years in West London.
Monaco also signed Tunisia centre-back Aymen Abdennour on loan from Toulouse with an option for a permanent deal.
The 24-year-old, who has 31 caps, is widely regarded as one of the most reliable defenders in Ligue 1.