Mark Rothko, a master of Abstract Expressionism, is known to have created 835 paintings during the course of his five-decade career. Christie's claims it just found No. 836. The auction house said it has discovered a previously unknown Rothko painting, which it expects to sell for at least $18 million on May 11 in New York. The 7-foot-long canvas, "Untitled #17," dates to 1961 and depicts a pair of fuzzy-edged pink and red rectangles against a gold background. The auction house said the work's American owner bought the piece directly from the artist in 1965 and kept it out of public view until now. Christie's specialist Jonathan Laib said the owner is selling now in part for estate-planning reasons and in part because he believes the painting will appreciate due to the rebounding art market overall. Christie's would not reveal the name of the owner. The discovery of a new Rothko is rare, especially a large canvas dating to 1961—the same year that the Museum of Modern Art gave the artist a lauded retrospective, said Marc Glimcher, the president of New York's Pace Gallery, which represents the Rothko estate. Mr. Glimcher said he and the artist's heirs, his two children, were not aware the work even existed. "It's new to us," Mr. Glimcher said. The rosters of artists' works are never written in stone: Two years ago, a 19th century German portrait of a young woman that once sold at auction for $19,000 was reattributed to Leonardo da Vinci, upending its potential resale value. The year before that, a rediscovered drawing by Dutch Old Master Lucas van Leyden was bought by the Metropolitan Museum of Art. Careful bookkeeping tends to keep better track of modern masters like Rothko, but pieces can still sit unnoticed, particularly if collectors maintain low profiles or rarely exhibit their pieces. To vet this new work's authenticity, Christie's said it has turned to David Anfam, a London-based art historian who oversaw the official tally of Rothko's painting output (called a catalogue raisonné) in 1998. Mr. Anfam said he inspected the work in person a month ago and deemed it a real Rothko because its size, palette, and brushwork are "consistent with Rothko's hand and whole way of painting." Mr. Anfam added that the artist's penciled signature on the back of its wooden frame also appears to be a match. Christie's said it has also informed the National Gallery of Art, which is cataloguing the artist's works on paper. A museum spokeswoman confirmed that its Rothko experts know about the painting's discovery but are not permitted to weigh in on the merits of anything earmarked for auction. Rothko, a Latvian immigrant to New York, died in 1970. His works, which were popular with collectors during his lifetime, also soared at auction before the recession, thanks to fresh attention from wealthy collectors in Russia and the Middle East. At the market's peak in 2007, one of his 1950 works sold for a record $72.8 million. None of his paintings have topped $51 million since then, but auction houses continue to test the upper reaches of his market.