BIRDscapes has been running from Glandford, near Holt, since October 2005.
It was launched by the conservationist and broadcaster Stephen Moss and his partner Liz Harris.
They met at university and moved to Norfolk in 1974 because it was “good for birding”.
The pair have always loved contemporary wildlife art – so, when they were offered a barn on the Bayfield Estate in 2004, they decided to transform it into their own gallery.
They are now looking towards semi-retirement and are gradually handing the reins to their children, Emma and Pete, who will be helped by long-term gallery assistant Julie.
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But first, they will host a special exhibition to celebrate their 20th anniversary.
Opening at midday on Saturday, it will showcase work by prominent artists such as Robert Gillmor, Michael Warren and Martin Woodcock, who all helped mentor the founders in the early days of the gallery.
Author and naturalist Nich Acheson will also speak at the exhibition and guests will be able to see the largest painting the gallery has ever shown – a 3.6m wide, life-sized painting of the head of a sperm whale by Norwich artist Tom Ashworth.
Ms Harris said: “When the doors of BIRDscapes first opened in 2005, we had no idea where it might lead.
“We never could have believed that in 20 years’ time it would be one of the UK’s leading wildlife art galleries.
“We wanted to bring contemporary, dynamic and environmental bird and wildlife art to wider attention and were honoured by the very positive response we received from so many great wildlife artists.
“Several customers have told us that BIRDscapes is like a well-being centre – they come in feeling stressed and go out calm, uplifted and inspired.
“This makes all the hard work worthwhile.”