The executor of Cornelius Gurlitt's estate discovered the French Impressionist artwork in a suitcase handed over to him by the clinic this week.
"The work on paper shows a landscape in light blue," the government task force investigating the hoard said in a statement.
The executor informed a court in the southern city of Munich of the findings, the task force said.
"An initial look through the Monet catalogue of works indicates that it may have been completed in 1864," given its similarity to the painting "Vue de Sainte-Adresse" finished that year.
Gurlitt had hidden 1,280 paintings, drawings and sketches — believed to be worth hundreds of millions of dollars and including masterpieces by Picasso and Chagall — in his Munich flat for decades.
Many of the works, which were seized in early 2012 when they were discovered by chance during a tax evasion probe, are believed to have been stolen or extorted from Jews under a Nazi scheme to systematically plunder valuable art collections.
Gurlitt, who never married or had children, described the works in an interview with a German magazine as "the love of his life".
Before his death at the age of 81, Gurlitt struck a deal with the German government to help track down the rightful owners of the artwork.
In the course of its investigations, the task force has announced spectacular new finds including sculptures thought to be by Degas and Rodin uncovered in Gurlitt's cluttered flat in July.
A day after Gurlitt's May 6 death, Switzerland's Museum of Fine Arts in Bern said it had been astonished to learn it was named as the recipient of Gurlitt's collection in his will, an offer it said it was assessing.
During the Nazi era, Gurlitt's father Hildebrand was tasked with selling works taken or bought under duress from Jewish families, and avant-garde art seized from German museums that the Hitler regime deemed "degenerate".
Experts have determined that around 450 works in the Gurlitt collection are suspected of being looted art, while another 380 may have been confiscated "degenerate" works.