Sir Roger Williams (1539–1540 – 12 December 1595) was a Welsh soldier of fortune and military theorist, who served the Protestant cause, fighting against the Spanish in several theatres of war.

Though the campaign was not a success, Leicester wrote: "Roger Williams is worth his weight in gold, for he is noe more valiant than he is wise and of judgement to govern his doings".

He met the Earl of Essex in 1589, and the two became close when they were sent by Queen Elizabeth to lead an army as allies of Henry of Navarre against the Catholic League during the Spanish occupation of Brittany in 1592.

[8] After Essex returned to England Williams became overall commander of the English forces supporting Henry and de facto envoy of the Queen.

During the Siege of Rouen in 1592, he almost killed Albanian-Italian leader Giorgio Basta in personal combat, slicing his neck and driving him and his men from the field.

[11] He returned to England permanently in 1594, but broken in health he died the following year and his death elicited a great show of public mourning.

[5][12] Williams was recognised as an expert on military matters by his contemporaries, and wrote A Brief Discourse of War, with his opinions concerning some part of Martial Discipline (1590).

[2] Williams gave detailed analyses of the different methods and effectiveness of the Dutch and Spanish forces in the Netherlands, commending the discipline of Parma's troops.

Williams has been called a "fiery Welshman", who was "rough around the edges, bloody minded, often lacking in self-control and, at times, slightly mad", but he was widely regarded as "a soldier of the highest integrity".

[13] Sidney Lee says that "His letters and literary work prove him to have possessed command of a blunt and forcible vocabulary as well as much sagacity as a student of the art of war.

Sir Roger Williams (who was a Welchman, and but a taylour at the first, though afterwards a very brave souldier) being gracious with Queen Elizabeth, prefer’d a suite to her, which she thought not fit to grant; but he, impatient of a repulse, resolv’d to give another assault; so coming one day to court, makes his address to the Queen, and watching his time, when she was free and pleasaunt, began to move again; she perceived it at the instant, and observing a new pair of boots on his legs, claps her hand to her nose and cryes, "Fah, Williams, I prythee, begone, thy boots stink".

[14]The story includes the implausible assertion that Williams was a tailor before he was a soldier, which may result from a confusion about the pun on "suit[e]", but Lee notes that his family, though members of the gentry, were relatively poor.

[17] E. K. Chambers accepted that "Wilson has made a fair case for finding, traits of the Welsh soldier Sir Roger Williams in Fluellen".