| Honorific Prefix: | His Grace |
| The Duke of Somerset | |
| Succession: | Duke of Somerset |
| Reign: | 1671–1675 |
| Reign-Type: | Tenure |
| Predecessor: | William Seymour |
| Successor: | Francis Seymour |
| Birth Name: | John Seymour |
| Birth Date: | born 1646 |
| Father: | William Seymour |
| Mother: | Frances Devereux |
John Seymour, 4th Duke of Somerset and 3rd Marquess of Hertford (before 164629 April 1675) was an English peer and MP.
| Short Title: | Restoration of Marquis of Hertford Act 1660 |
| Type: | Act |
| Parliament: | Parliament of England |
| Long Title: | An Act for the restoring of the Marquis of Hertford to the Dukedom of Somersett. |
| Year: | 1660 |
| Citation: | 12 Cha. 2. c. 10 |
| Royal Assent: | 13 September 1660 |
| Collapsed: | yes |
He was the only surviving son of William Seymour, 2nd Duke of Somerset, and Lady Frances Devereux. He married Sarah, daughter and co-heiress of Sir Edward Alston and widow of George Grimston[1] in exchange for a dowry of £10,000. Her father negotiated an independent income of £300 per year for his daughter and additional clauses to cover the possibility that she became a widow. The marriage was a financial success but otherwise it was a disaster. William liked drinking and gambling, but Sarah did not.[2]
He was elected Member of Parliament for Marlborough in 1661 and entered Gray's Inn in 1666. He succeeded his nephew as the 4th Duke of Somerset in 1671.
He spent the last two separated from his wife after she petitioned the King concerning his behaviour before he died in 1675, childless.[2] He was buried in Salisbury Cathedral. He was succeeded in the dukedom by his cousin, Francis Seymour. The Marquessate of Hertford became extinct on his death.[3]