The family foundation of the international shipping magnate Eyal Ofer has donated £10 million to Tate Modern to help fund its development.
This means the planned £215 million development, which will provide more gallery space, is now 85 per cent funded.
The exhibition galleries on Level 3 East will be named the Eyal Ofer Galleries following the donation.
The expansion scheme also includes plans to build a bridge across the Turbine Hall in the south London gallery on the banks of the Thames.
Mr Ofer, whose father Sammy was a major contributor to the arts and helped fund the restoration of the Cutty Sark in Greenwich, said the gallery was "one of the world's great public art institutions".
He said: "It is my privilege to be supporting this development project as it expands the gallery's exhibition space and enables an iconic institution to enhance the experience and accessibility of contemporary art for an even broader range of visitors from across the UK and abroad."
Tate director Nicholas Serota said: "I am delighted that the Eyal Ofer Family Foundation has chosen to make such a major contribution towards Tate Modern's future.
"It is exciting to see such outstanding philanthropy continuing from one generation to the next. The generosity of Eyal Ofer and his family will help to make Tate Modern a truly 21st-century museum."