The painting, by Sir Anthony van Dyck, was purchased by historian Christopher Wright in the 1970s and hung in his sitting room for decades.
He originally thought it was a copy of Van Dyck’s, but to his surprise it has recently been confirmed as an original.
As a friend of Barnsley Museums, Christopher wants the painting to be seen by many and has loaned the artwork to the museum.
Coun Tim Cheetham, cabinet spokesperson for place, said: “It will be exciting for the museum to reopen with such an exquisite piece of artwork on display.
“Never seen before, it’s a great opportunity for the painting to be enjoyed by the thousands of visitors.
“We would like to thank Christopher for his generosity in loaning the portrait to the museum.
“Cannon Hall is temporarily closed for annual maintenance, but they can’t wait to welcome visitors from far on wide when they reopen once again in February.”