British art collector Simon Sainsbury, who died last year, bequeathed 18 paintings from artists including Claude Monet, Paul Gauguin and Lucian Freud.
Tate director Nicholas Serota called it "one of the most important bequests" to the nation in the last 100 years.
The Tate will exhibit the 18 donated paintings in a show next summer.
The works will then be divided up and the Tate will receive 13 pieces and the remaining five will be shown at the National.
Martin Wyld, acting Director for The National Gallery said: "Simon Sainsbury's legacy at the National Gallery is truly remarkable.
"He long intended that major works from his distinguished collection should come to the National Gallery and these outstanding paintings will greatly enrich our Impressionist and Post-Impressionist collection."
Sainsbury, a private but influential arts and heritage benefactor, was from the family behind J Sainsbury Plc, one of Britain's biggest supermarket chains.
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