nico Willem van Wassenaer, Count of the Empire, (2 November 1692, Delden, Overijssel – 9 November 1766, The Hague) was a Dutch diplomat and composer.

His most important surviving compositions are the Concerti Armonici, which until 1980 had been misattributed to the Italian composer Giovanni Battista Pergolesi (1710-1736) and to Carlo Ricciotti (1681-1756).
Van Wassenaer was born into a distinguished family of wealth, power and accomplishment--the House of Wassenaer. His grandfather, Jacob van Wassenaer Obdam, was supreme commander of the confederate Dutch navy, and was killed in battle in 1665 in the Second Anglo-Dutch War; his father, Jacob van Wassenaer Obdam (younger), served as a general of the army in the War of the Spanish Succession, was defeated in 1703 in the Battle of Ekeren, and spent the rest of his career as a diplomat.

Van Wassenaer studied law and other subjects at the University of Leiden and, in 1723, married Dodonea Lucia van Goslinga (the daughter of Sicco van Goslinga), with whom he had three children. Van Wassenaer occupied high diplomatic, military and commercial posts, representing his country at the Congress of Breda. It is known that he was respected not only as a diplomat, but also as a musician.
Between 1725 and 1740, he wrote the Concerti Armonici, but he did not publish them in his own name, because he was a noble man or because he doubted his ability as a composer.[citation needed] The concertos were published in 1740, by the Italian violin player Carlo Ricciotti (1681–1756), to whom the concertos were attributed at first. Van Wassenaer has therefore been called "the mystery composer."

The Polish composer Franciszek Lessel (1780–1838) asserted incorrectly that the concertos were written by Pergolesi. Since the style of the concertos is Italian, laid out in typical Roman fashion with four parts for violin and consisting of four parts instead of the Venetian three, they are comparable to works by Pietro Locatelli.

However, in 1979-1980 a manuscript of the six concerti was found in the archives of Twickel Castle (the castle where Van Wassenaer was born)[1] labeled "Concerti Armonici." Although the handwriting was not by Van Wassenaer, the manuscript did have an introduction in his hand, reading: "Partition de mes concerts gravez par le Sr. Ricciotti". Because of the research done by the Dutch musicologist Albert Dunning, there can be no doubt that the concerti were, in fact, written by Van Wassenaer.