The actor's gift, announced with delight by the Trust last night, goes a long way to meeting the pounds 3m-plus price of the two mountain estates, one including the 3,560ft summit, which the Trust agreed to buy last week.
Sir Anthony, 60, is Britain's highest-paid performer and last month agreed to make a sequel to the film which brought him Hollywood superstardom and a 1991 Oscar - The Silence of the Lambs. For his second appearance as the cannibal psychiatrist, Dr Hannibal Lecter, he is to receive a fee of pounds 15m.
Although now resident in California, he was born in Port Talbot in Wales and has for some years been president of the National Trust's Snowdonia Appeal.
"Snowdonia is one of the most beautiful places in the world and Snowdon is the jewel that lies at its heart," Sir Anthony said. "It must be cherished and protected ... I am very pleased that I am able to offer this support."
The Trust was "totally overwhelmed" by Sir Anthony's generosity, Richard Cuthbertson, chairman of its Committee for Wales, said last night.
Later this month, the Trust said, the actor will make a trip to Snowdon to see the estates.
The Trust has another 93 days to find the purchase price - thought to be about pounds 3.5m - of the Hafod-y-Llan and Gelli Iago estates on the southern flank of the mountain, which it agreed to buy from Richard Williams, whose family has farmed the land for 14 generations.