The artist created the mask in 2002 as a "specimen" or "museum display" to "transform herself into an object of scrutiny for generations to come".
Emin and her gallery White Cube priced the mask at Â£60,000 - but offered the National Portrait Gallery a Â£30,000 museum discount.
The remaining Â£30,000 came from a grant from The Art Fund.
Death masks have been created throughout history to preserve the final images of famous people just after they have died.
Emin is very much alive - and her bronze cast is now in its "rightful home", according to the National Portrait Gallery.
Associate curator Rab MacGibbon said: "Artists have frequently explored their mortality in self-portraits. Tracey Emin's Death Mask... blurs the distinctions between life and death, art and identity."
Death Mask will feature in the exhibition Life, Death And Memory, which also includes the death mask of painter John Constable and the last portrait for which film director and artist Derek Jarman sat.
Emin, 53, is best know for her autobiographical works such as My Bed and the tent Everyone I Have Ever Slept With.
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Tracey Emin has helped the National Portrait Gallery buy one of her more unusual works - her own "death mask".