Sir Edwin Manton has given the gallery pounds 7m, and promised another pounds 5m in his will. In addition, he will be leaving the Tate a recently discovered Constable painting, The Glebe Farm.
The British-born 88-year-old, who has lived in New York for 60 years, where he is estimated to have amassed a pounds 260m fortune, visited the Tate as a young boy and began collecting art works in 1945.
Sir Edwin, who - together with Prince Charles - was in London for last night's celebrations to mark the Tate's centenary, said: "There is some method to my madness. I have lived abroad for many years but I'm a patriotic Englishman."
Born in Essex, and educated at Shaftesbury Grammar School and London University, he said: "It may sound silly but I feel I kind of owed the country something. I was born in Earls Colne, about 20 miles from Constable's birthplace, and I feel nostalgic."
Sir Edwin was knighted in 1994 for his services to the gallery, but insisted he remain anonymous.
Nicholas Serota, the Tate's director, explained why Sir Edwin's identity was finally revealed a fortnight ago. "He's always very modest and has not wanted his name up in lights, but with the Tate's centenary he agreed that we publicise his identity."
Sir Edwin added: "I wanted to be anonymous to protect myself from people importuning me. I may not be the wealthiest person in America but I was protecting my purse. It was not a noble feeling."
He bought his first "Constable" in 1945 but was dismayed to learn that it was by a German artist. He has since collected 50 genuine Constable sketches, oils and watercolours.
"I am deeply indebted to the Tate for taking me in. I won't buy a Constable without asking Leslie Parris [the Tate's deputy keeper of the British collection] if it's a good one."
Sir Edwin's donation to the Tate has ensured that a pounds 31m redevelopment of the Millbank building can begin later this year. Mr Serota suggested his contribution would be recognised by naming a part of the building after him.
"Sir Edwin's gift will allow us to transform the way we show British art."Reuse content