The Renaissance master's dreamily sensual sketch of Saint Sebastian was classified as a French national treasure on December 29th, Tajan auctioneers in Paris said during an unveiling of the work to the press.
The French government has a “right of pre-emption” under which it has 30 months to purchase items that it deems should remain in France.
Discovered in the papers of a French provincial doctor who wants to remain anonymous, the drawing measures 19.3 x 13 centimetres (7.6 x 5 inches) and was authenticated by leading Da Vinci experts.
The dramatic study, which it is thought Leonardo did in his late 20s or early 30s, is one of eight he is known to have drawn of Saint Sebastian.
It is thought the sketch, done with a quill pen on thin paper, may have come from one of the artist's famous notebooks.
A couple of scientific sketches are on the back, as well as some back-to-front writing, a technique Da Vinci used regularly so that his writing could be read only by using a mirror.
The find is extremely rare, with the last Da Vinci drawing that came to market — a sketch of a horse and rider — equalling the world record for an Old Master drawing when it sold for $10 million in 2001.