Claude Nau or Claude Nau de la Boisseliere (d. 1605) was a confidential secretary of Mary, Queen of Scots, in England from 1575 to 1586. He was involved in coding Mary's letters with cipher keys.

Nau was a successful lawyer practising in Paris. He was recruited by the Guise family in 1574 to be Mary's secretary. Jean Champhuon, sieur du Ruisseau, an advocate who married Nau's sister Claire in 1563, also joined Mary's service. An account of the death of Mary, Queen of Scots, mentions that Ruisseau was Claude Nau's brother-in-law, a beau frere, and Albert Fontenay was Claude Nau's brother or half brother.

Nau was presented by the Duke of Guise, Mary's nephew, to Henri III of France. The King gave him diplomatic accreditation and sent him to Elizabeth I of England. On 29 March 1575, Elizabeth gave him a letter of introduction to the Earl of Shrewsbury the Scottish Queen's keeper at Sheffield Castle. Nau was a replacement for the secretary Augustine Raullet. He was known to Mary's ally in France, James Beaton, Archbishop of Glasgow. Nau was frequently mentioned in Mary's correspondence, and many of his own letters survive. In January 1577, Nau sent cipher code keys to his brother-in-law the treasurer Jean de Champhuon, sieur du Ruisseau, to Mr Douglas, to John Lesley, Bishop of Ross, and to Ralph Lygon, for use in their correspondence with Mary.

In August 1577 Nau added a postscript to one of Mary's letters to her ally in France, James Beaton, Archbishop of Glasgow, that he intended to send him the queen's portrait, but the painter working at Sheffield Castle had not completed the work to perfection. At this time, Mary was contemplating marriage with John of Austria, a brother of Philip II of Spain, and the Archbishop was her negotiator. Although the artist Nicholas Hilliard had painted Mary's portrait, at this time he was in France. A surviving miniature portrait of Mary, in a later setting, the Blairs jewel, may date from this period and is associated with Elizabeth Curle, the sister of Mary's Scottish secretary Gilbert Curle.

In the same month, Claude Nau wrote twice to his brother, du Ruisseau, using cipher codes. He hoped that du Ruisseau could be promoted to be treasurer of Mary's French dowry in place of René Dolu, and that du Ruisseau would speak to his own advantage at the French court. Nau also asked him to buy some jewellery: a locket with a catch or a sealed box (une petite boite fermee et cachetee), a pair of bracelets made in the latest fashion, and a diamond or emerald shaped like a heart or triangle. A case for a miniature portrait was sometimes known as a "picture box" in English, as une boîte à portrait in French, and in Spanish, a caxa or caxilla, although Nau was probably referring to the packaging of the bracelets and stone "closed-up in a small box under seal". Nau advised that the precious stone would cost less from a specialist lapidary than from a goldsmith, and prices were cheaper because of the wars in France. Mary considered other candidates to replace Dolu in October 1579, including the father-in-law of the writer Adam Blackwood.

In June 1579, Mary sent Nau as her ambassador to her son, James VI of Scotland, instead of John Lesley, Bishop of Ross. She wrote to George Bowes at Berwick-upon-Tweed at the border asking him to assist Nau's journey. However, the Scottish court at Stirling Castle would not allow him an audience, apparently because Mary's letter was addressed to her son, not the King. Although Francis Walsingham had directed Nicolas Errington, Provost Marshal of Berwick, to accompany Nau to the Scottish court, he had no papers from Elizabeth. The Privy Council of Scotland issued a proclamation that he deserved punishment and should be commanded to depart. Errington reported that few spoke to Nau except old servants requesting payment.

Mary mentioned in a letter to the Countess of Atholl that Nau had brought "tokens", gifts for her supporters, and a picture and a book for James. Some accounts say that Nau tried to deliver a gift of valuable jewels and a vest that she had embroidered for James. The chronicle known as The Historie of James the Sext describes the gift as "certain jewels and ornaments to his body". Errington wrote to Francis Walsingham that Nau had been unable to deliver Mary's tokens. Nau wrote that he had not been allowed to see James VI in a postscript to Mary's letter of 4 July 1579 to the Archbishop of Glasgow.

Mary wanted Nau to go to Scotland again in 1582, and asked the ambassador Michel de Castelnau to get permissions. Claude's brother-in-law, the Sieur de Fontenay, sent from France, had more success. Fontenay was able to meet James VI in August 1584. Fontenay wrote to Claude Nau about his good reception, James had met him in his cabinet at Holyroodhouse, and lent him a horse to join the hunting at Falkland Palace. On 15 November 1584, Nau came to London as Mary's ambassador and was lodged in a house belonging to Ralph Sadler. He spoke with Elizabeth, on the subject of Mary's allegations against Bess of Hardwick. Mary wanted Bess of Hardwick and her sons to acknowledge before the French ambassador that rumours about her were untrue. Nau also hoped to put forward the idea of the "association", a scheme to return Mary to Scotland as joint ruler with her son. However, James VI and another Scottish diplomat, the Master of Gray, made it known that James was not about to accept joint rule. Nau was informed of plans to move Mary to another lodging, at Tutbury Castle.