The 500 lb explosive was discovered 15 yards underwater in Lake Como just in front of the actor's 30-bed mansion, Villa Oleandra, Daily Telegraph online reported.
Apart from the bomb, thought to be an ordinance dropped from a WWII bomber, Italian police said they also found mines and other raw explosives.
Clooney is not staying at the villa at the moment.
In February the Oscar-winning actor announced plans to sell the villa sparking a bidding war between David and Victoria Beckham and a Russian vodka billionaire.
Clooney bought the opulent villa in 2001 for a mere seven million euros which now costs 30 million euros. It was reported he hoped to move to a quieter location with his Italian girlfriend Elisabetta Canalis.
Many celebrities have or had homes on the shores of Lake Como, such as Matthew Madonna, Gianni Versace, Ronaldinho and Sylvester Stallone.
Lake Como is widely regarded as being one of the most beautiful lakes in Italy.
Published 30 April 2010, 12:16 IST