Stephen Lesieur or Le Sieur (fl. 1575 –1640) was a Swiss-born English ambassador to Denmark, Florence, and the Holy Roman Empire.
In March 1583 an envoy in London collecting money for the church in Geneva, Jean Maillet, met Lesieur and they discussed efforts to raise a ransom for the English diplomat Daniel Rogers, who had been captured by Maarten Schenck van Nydeggen.
After the Union of the Crowns in 1603 it was said Lesieur would be sent to announce King James' accession the throne of England in Germany while Anthony Standen was sent to Italy. He was ambassador to the Holy Roman Empire in 1603-1604 and 1610-1614, and ambassador to Florence in 1608-1609.
On 12 February 1603 Lesieur wrote from Bremen to Christian IV of Denmark, sending a miniature portrait of Queen Elizabeth that he had commissioned in London at Christian's request. James VI and I paid him in June 1603 for his role as assistant ambassador to Denmark for Queen Elizabeth, at a rate of forty shillings a day. On 17 August he wrote from Neukloster near Wismar to Robert Cecil detailing his movements and meetings, following his audience with Charles I, Duke of Mecklenburg.
In March 1608 he was preparing to go to Italy when one of his companions and cousins, the eldest son of Sir Richard Norton was challenged to a duel by Henry Clare (a follower of the Earl of Montogomery) for wrongs done to his sister. Lesieur wrote to the Earl of Salisbury to prevent a fight. The young man, later Sir Richard Norton of Rotherfield (d. 1646) joined the embassy in Florence.
The Governor of Vlissengen, John Throckmorton heard in October 1612 that Lesieur had a "sour" audience with the Emperor, and a better reception from Archduke Maximilian, and had gone to the Duke of Brunswick to offer condolences on the death of his father.
In May 1614 Elizabeth Stuart, Queen of Bohemia wrote to her father King James saying that Lesieur could explain the actions of Colonel Schönberg that would make him a suitable husband for her lady in waiting, Anne Dudley, daughter of Theodosia Harington and Edward Sutton, 5th Baron Dudley.
The dates of Lesieur's birth and death are uncertain. He lived at Chiswick in old age.
- Roger Kuin, Correspondence of Sir Philip Sidney, vol. 1 (Oxford, 2012), p. xlviii.
- Simon Adams & Mark Greengrass, 'Memoires by Jean Mailet', Religion, Politics and Society in Sixteenth-Century England (Cambridge, 2003), p. 183.
- Calendar State Papers Scotland, vol. 9 (Edinburgh, 1915), pp. 618-9.
- William Dunn Macray, 'Report on the Royal Archives in Denmark', 45th Report of the Deputy Keeper of the Public Records: Appendix II (London, 1885), p. 54
- Thomas Rymer, Foedera, vol. 16 (London, 1715), pp. 429.
- Edmund Lodge, Illustrations of British History, vol. 3 (London, 1791), pp. 164-5.
- William Dunn Macray, 'Report on the Royal Archives in Denmark', 45th Report of the Deputy Keeper of the Public Records: Appendix II (London, 1885), p. 56.
- Frederick Devon, Issues of the Exchequer: Pell Records (London, 1836), pp. 3, 137, 175.
- HMC Salisbury Hatfield, vol. 15 (London, 1930), pp. 234-6.
- HMC Salisbury Hatfield, vol. 20 (London, 1968), pp. 110-1.
- William Shaw & G. Dyfnallt Owen, HMC 77 Viscount De L'Isle Penshurst, vol. 5 (London, 1961), p. 131.
- Nadine Akkerman, The Correspondence of Elizabeth Stuart, Queen of Bohemia: 1601-1631, vol. 21 (Oxford, 2015), p. 140.
- Roger Kuin, The Correspondence of Sir Philip Sidney, vol. 1 (Oxford, 2012), pp. xxii-xxiv.