The painting by Bill Leak has been on temporary loan to the gallery since 1999.
But Mr L Gordon Darling today donated the portrait to the national collection, coinciding with the centenary of Bradman's birth.
'27 August 2008 marks yet another century for 'The Don'. For me it's the perfect time for my sporting hero to enter our national collection,' Mr Gordon said.
National Portrait Gallery director Andrew Sayers says it is an important acquisition.
'It is impossible to imagine an Australian National Portrait Gallery without a portrait of Bradman,' he said.
Bill Leak painted the portrait from life in 1990.
'[Leak] included in it a subtle visual allusion to Bradman's elegance and effectiveness with the bat during his playing career,' Mr Sayers said.
'Leak is one of our best political cartoonists but in this portrait we see him in serious portrait mode, paying tribute to a towering figure in Australian sporting history.'
Leak says he has very fond memories of Bradman.
He says he took his father - who had admired Bradman his whole life - to meet the legend.
'They clicked and hit it off instantaneously and we stayed and had a cup of tea and had a lovely afternoon,' he said.
'They spoke the same language and they sat there and laughed all afternoon and had a really great time.
'Afterwards [my father] said to me on the way home, 'well Bill, that was the best day of my life'.'
Leak says despite Sir Donald's reputation as a private person, he was at ease with his fame.
'It's funny because I was warned that he would be very aloof and that he wasn't a very humorous person and that I might find him difficult and a bit brittle,' he said.
'But in fact my experience of him was quite the opposite - we laughed a lot, he told me some very funny stories.'
The Bradman portrait will feature in the Inaugural Hang in the new National Portrait Gallery building opening in Canberra in December.
Image and text by ABC News, Australia