The Rev. Jamie MacLeod, who runs a retreat house in Derbyshire, took the painting to a show in June in Cirencester, the BBC reported.
Fiona Bruce, the show's host, saw the photo and thought it might be genuine.
The photo underwent a restoration and Christopher Brown, an expert on van Dyck, verified the painting.
"It's been an emotional experience and it's such great news," MacLeod said.
The artwork is the most valuable painting identified in the show's 36-year history, the BBC said.
Van Dyck was the leading court painter for King Charles I, and the painting is a portrait of the Magistrate of Brussels.
The painting was likely completed in 1634.
"It's everyone's dream to spot a hidden masterpiece, I'm thrilled that my hunch paid off, to discover a genuine van Dyck is incredibly exciting. I'm so pleased for Father Jamie," Bruce said.
MacLeod said he is planning to sell the artwork to buy new church bells.
The painting will be featured on Antiques Roadshow at 7 p.m. GMT on Sunday on BBC One.