The Art Loss Register, which tracks stolen, missing and looted art, says "Le Jardin" was found when a British dealer checked the picture against the group's database before selling it.
Dealer Charles Roberts, who runs Charles Fine Art in southern England, said he was shocked to discover the painting was stolen.
"It's not something that happens every day," Roberts said. "I'm glad I found out now rather than later."
Roberts said the current Polish owner, whom he did not name, had bought the Impressionist artwork in good faith 20 years ago.
Christopher Marinello, a lawyer working with the London-based Art Loss Register, said the 1920 painting, valued at about $1 million, would be returned to Stockholm's Moderna Museet. It was snatched from the gallery during a nighttime raid in May 1987.
The museum's director, Daniel Birnbaum, said it was "extremely gratifying that the painting has surfaced after so long."
"We are reassured that the painting appears to be in good condition and look forward to having someone from the museum staff look at it," he said.
Birnbaum said he was in contact with the Swedish Culture Ministry about the formal return of the painting.
Marinello said it was up to Swedish police to decide whether to try to track down the thieves.