Sir John Spencer III, Knight
- Born: 1546, Althorp, Daventry District, Northamptonshire, England
- Marriage (1): Mary Catlyn
- Died: 9 Jan 1600, Northamptonshire, England at age 54
- Buried: St Mary the Virgin with St John Churchyard, Great Brington, Daventry District, Northamptonshire, England
Sir John Spencer, Kt, was the eldest son of Sir John Spencer, Kt, of Wormleighton & Althorp and Katherine (Kitson). He Married Mary Catlin, daughter of Sir Robert Catlyn, of Berne, Dorset. To this union one son was born; Robert 1st Baron Spencer, of Wormleighton.
Sir John Spencer was invested as a Knight in 1588 by Queen Elizabeth I, upon the death of his father.
Sir John planted a large wood a the rear of Althorp. The oldest oaks still growing there today were planted at his instruction in 1589. It was Sir John's intention to boost the timber stocks, in order to help enable the English Navy to build more ships.
Sir John increased the Althorp estate by buying Little Brington in 1592 from Francis Bernard.
John married Mary Catlyn. (Mary Catlyn was born in 1555 in Northamptonshire, England, died on 5 Jan 1619 in Northamptonshire, England and was buried in St Mary the Virgin with St John Churchyard, Great Brington, Daventry District, Northamptonshire, England.)