Completed just before the revolution in 1782 for a German prince by the architect Pierre Rousseau it was soon taken over by the new government for use by the Legion of Honor. Rousseau's best-known work today is this building but he was really the starchitect of the day, first for the ancien regime and then the governments which were to follow.
The entry court probably looks familiar to you as it has been copied in many forms in public buildings around the world.
This rare floorplan above (from the book The Architecture of the French Enlightenment by Allan Braham) is situated with the Seine towards the top of the image, what many people now consider to be the 'front' of the building despite no entry. The addition from the mid 1920s to the lower right now houses the main entry into the museum and other museum offices.
Very few tourists who visit the Musee d'Orsay (just to the right of the image above) know to turn the corner to catch a glimpse of the main entry and courtyard of the Hotel de Salm.
HERE, and also at the Chateau de Rochefort en Yvelines (a few images below). It was one of Thomas Jefferson's favorite buildings, aspects of which he incorporated into his own home at Monticello.
L'hotel de Salm published by Monelle Hayot further details the existing building and opulent interiors should you be interested.