A Serjeant-at-Law (SL), commonly known simply as a Serjeant, was a member of an order of barristers at the English and Irish Bar. The position of Serjeant-at-Law (servientes ad legem), or Sergeant-Counter, was centuries old; there are writs dating to 1300 which identify them as descended from figures in France before the Norman Conquest, thus the Serjeants are said to be the oldest formally created order in England. The order rose during the 16th century as a small, elite group of lawyers who took much of the work in the central common law courts.

With the creation of Queen's Counsel (or "Queen's Counsel Extraordinary") during the reign of Elizabeth I, the order gradually began to decline, with each monarch opting to create more King's or Queen's Counsel. The Serjeants' exclusive jurisdictions were ended during the 19th century and, with the Judicature Act 1873 coming into force in 1875, it was felt that there was no need to have such figures, and no more were created. The last appointed was Nathaniel Lindley, later a Law Lord, who retired in 1905 and died in 1921. The number of Irish Serjeants-at-law was limited to three (originally one, later two). The last appointment was A. M. Sullivan in 1912; after his 1921 relocation to the English bar he remained "Serjeant Sullivan" as a courtesy title.

The Serjeants had for many centuries exclusive jurisdiction over the Court of Common Pleas, being the only lawyers allowed to argue a case there. At the same time they had rights of audience in the other central common law courts (the Court of King's Bench and Exchequer of Pleas) and precedence over all other lawyers. Only Serjeants-at-Law could become judges of these courts until the 19th century, and socially the Serjeants ranked above Knights Bachelor and Companions of the Bath. Within the Serjeants-at-Law were distinct orders: the King's Serjeants, particularly favoured Serjeants-at-Law, and within that the King's Premier Serjeant, the Monarch's most favoured Serjeant, and the King's Ancient Serjeant, the oldest. Serjeants (except King's Serjeants) were created by Writ of Summons under the Great Seal of the Realm and wore a distinctive dress, the chief feature of which was the coif, a white lawn or silk skullcap, afterwards represented by a round piece of white lace at the top of the wig.

Although the Serjeants are extinct as a class of advocates, the title The Serjeant-at-Law in the Common Hall is still given to the judge generally known as the Common Serjeant of London.