Sir Thomas Sadleir (c. 1536 – 5 January 1607) of Standon, Hertfordshire was an English landowner and politician.

"[1] He married, firstly, Ursula Sharington, daughter of Sir Henry Sharington of Lacock, Wiltshire, with whom he had no children; secondly, Gertrude Markham, daughter of Robert Markham of Cotham, Nottinghamshire, with whom he had a son and a daughter:[1] On Saturday 30 April 1603, King James VI of Scotland, on his progress toward London to claim the English throne, came to Standon and having been met by the Bishop of London and a company of gentlemen "in coats and chains of gold," proceeded to Standon Lordship, where he stayed for two nights.

[10] "At Sir Thomas Sadleir's, his Majestie was Royally entertained, for himselfe and his Kingly Traine; nothing being wanting the best desired, nor that the meanest could demaund.

[13] An elaborate tomb, with recumbent effigies of Sir Thomas and his wife, has a Latin inscription above:[14]D. O. M.ETMemorise Thomæ Sadleiri, Equitis AuratiHie situs obdormit Christo, Christoque resurgatThomas Sadleirus stemmate Claris equesQuo micuere simul bonitas, prudentia, candor,Cum probitate pudor, cum pietate fides.Heec ilium decorant, bsec sunt monumenta sepultoQui Tumulo decus est et fuit ante suisAttamen hoc posuit monumentum filius illiUt constet pietas officiosa patri.

On the side of the monument, are effigies of his son and daughter on their knees, with an inscription between them:[15]"Here resteth in assured hope of resurrection in Christ, Sir Thomas Sadleir, of Standon, Knight, son and heir of the Right Honourable Ralph Sadleir, Knight Banneret, Privy Counsellor to three Princes of this land; which Sir Thomas lived in honourable reputation for his learning and all other virtues; and as he lived, he ended his life most Christianly, leaving Ralph and Gertrude his children, by his wife Gertude, daughter of Robert Markham of Cottham in the county of Nottingham, Esq.