William St Clair (died 1610) was a Scottish nobleman and the 15th Baron of Roslin.

He was the second son of William St Clair, 14th Baron of Roslin, who according to Alexander Nisbet's a System of Heraldry, had married Isabel, daughter of Ker of Cessford,[1] but who according to Father Richard Augustine Hay's manuscript of 1690, had married Lindeasy, daughter of the Laird of Egle, brother of the Earl of Crawford, with whom he had William.

[2][3] William St Clair, 15th Baron of Roslin was the first Hereditary Grandmaster of the Scottish Order of Freemasons which was constituted to him by charter.

[3] In 1601, he received a charter from Henry Saintcler the Provost of Roslin for the church lands.

[3] He married Jean or Janet Edmonstone, daughter of the Laird Of that Ilk, with whom he had a son, William St Clair, 16th Baron of Roslin.