The £34.6 million overhaul of the A-listed Victorian landmark has been hailed as an "outstanding example" of how existing public buildings can be reused in the face of climate change.
The project included the creation of a striking copper-clad rooftop extension where touring exhibitions, including a show which fashion designer Zandra Rhodes has just opened, can be staged.
The overhaul, which opened up vistas throughout the building and across the city centre, also allowed the gallery to increase the number of items on display from its own renowned collection from 300 before the project began to more than 1,000.
Visitors can find work by leading Scottish artists Henry Raeburn, Joan Eardley, Samuel Peploe, Rachel McLean, Bill Gibb and James Cromar Watt, as well as nationally and internationally-acclaimed artists including Barbara Hepworth, Francis Bacon, Tracey Emin and Claude Monet.
The Royal Incorporation of Architects in Scotland (RIAS), which runs the Andrew Doolan Best Building in Scotland Award, described the gallery overhaul as a transformation of one of Scotland’s leading cultural institutions and made it “more welcoming, more sustainable and more accessible”.
The gallery project was honoured ahead of the transformation of a former egg shed in Argyll into a new community centre and visitor attraction, a new Edinburgh University building for research into artificial intelligence, a huge chainmail box created to protect a Charles Rennie Mackintosh masterpiece in Helensburgh, and a new national sports training centre in Inverclyde.
Previous cultural projects to win the ‘Doolan Prize’ include an overhaul and expansion of the world’s first Andrew Carnegie library in Dunfermline and the conversion of a former tenement block in Glasgow into new studio spaces for artists and creative businesses.
Hoskins Architects were honoured in 2011 for the transformation of the National Museum of Scotland’s original Victorian building in Edinburgh.
Aberdeen Art Gallery, which first opened in 1885, was closed for four years from 2015 for its biggest ever overhaul.
Christina Gaiger, president of the RIAS, said: “Aberdeen Art Gallery is an outstanding building and a highly deserving winner.
"Hoskins Architects have brought a piece of Scottish heritage into the 21st century with humility, skill and sensitivity. In the face of the climate emergency, how we upgrade, respect and adapt our existing building stock is absolutely crucial.
"In Aberdeen Art Gallery we have an outstanding example of how a public building, thanks to talented architects and far-sighted clients, exemplifies the smart re-use of an existing building, as part of a collective regenerative response to climate change.”
Hoskins Architects director Chris Coleman-Smith said: “The Doolan Award is the most important architectural prize in Scotland and we’re thrilled that Aberdeen Art Gallery has been recognised as the best building.
"The team has done an exceptional job of subtly and sensitively restoring original features of the 19th-century building and improving fabric performance, alongside confident alteration and the bold addition of new elements that enhance the visitor experience, knitting together a thread of careful conservation and the requirements of a world class, 21st-century gallery.
“This special award is testament to the effort and commitment from the many people involved in the design and delivery of this fantastic project throughout the years.”