Mervyn Tuchet, 4th Earl of Castlehaven (died 2 November 1686) was the third son of Mervyn Tuchet, 2nd Earl of Castlehaven, and his first wife, Elizabeth Barnham (1592 - c. 1622).

[1] He succeeded his brother James Tuchet as Earl of Castlehaven on 11 October 1684.

He also held the subsidiary titles 14th Baron Audley and 2nd Baron Audley of Hely.

He married Mary Talbot (buried 15 March 1710/1 in Clewer, Berkshire), 3rd[2] daughter of John Talbot, 10th Earl of Shrewsbury[3] (before 1601 - 1654) and his wife, Mary, daughter of Sir Francis Fortescue.

[1] She was the widow of Charles Arundel, Esq,[1] son to William Arundell second son to Thomas Arundell of Wardour.