The property is being restored, and parts of the building, which have been closed for 120 years, will be reopened, creating 50% more space.
The building on Edinburgh's Queen Street is to reopen in November 2011 after two years of work.
The world's first purpose-built portrait gallery opened in 1889.
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Director general of the National Galleries of Scotland, John Leighton, said the refurbishment had been one of their top priorities.
Leighton commented: "The project is fully on track and the building is set to reopen on time later this year. The transformation within the building is simply stunning and we are delighted that this boost in funding from the government will now enable us to concentrate on raising funds for the new programmes, activities and displays at the Portrait Gallery."
Culture Minister, Fiona Hyslop, said the money would be an important part of ensuring the redevelopment of the world's first purpose-built portrait gallery.
Hyslop noted: "We have been a strong supporter of the redevelopment of the Portrait Gallery and I am pleased to commit this additional support to complete the building work, enabling the National Galleries to move to their next fundraising phase."
(JG)
