But it may have just been found -- and in the external wall of the very gallery from which it was thought pilfered.
Investigators said they're remaining cautious as they await the results of high-tech X-rays to verify the work's authenticity, after which questions on how it went missing and who was involved still require answers.
The theft of "Portrait of a Lady" was believed to have happened during a three-day span leading up to Oct. 22, 1997, when the work was being moved into another building as part of a temporary exhibition.
The painting, one of Italy's most sought-after, has received immense attention ever since its disappearance. Years of investigations followed.
Custodians at the gallery were among those initially questioned, but the case eventually was abandoned because of a lack of evidence.
The case was briefly reopened in 2016 -- a trace of DNA was found on the painting's frame, which was discovered inexplicably on the gallery's roof -- but that investigation stalled.
The painting, from 1917, was part of the original donated collection when the Ricci Oddi gallery was inaugurated in 1931, in Piacenza, south of Milan.
What's believed to be the painting was found Tuesday afternoon by gardeners working on the gallery's exterior. Clearing away ivy from a wall, they discovered a small door that opened into a small storage space -- similar to those containing utility meters -- and found the artwork inside a large, black garbage bag.
Dario Gallinari, a gallery employee, told ABC News he first thought the gardeners were teasing him when they alerted him to their find. But Gallinari said he quickly changed his mind when he saw a corner of the painting peeking out from the garbage bag.
He took it into the gallery’s ticket office and called the police.
"My hands were trembling when I held the painting," he added. "It was an incredible to think that this could be possible."
Gallinari declined to speculate on how the painting could've ended up in an unused cubby hole, but did add, "All I can say is that it seems in perfect condition."
Gallinari was 9 when the painting went missing.
"The people of Piacenza were very distraught when the painting was stolen," he said, "and it will mean a lot to us all when the painting is put back where it belongs."