Alexander St Clair (1672–1706) was the 19th Baron of Roslin.

He was the second son of James St Clair, 18th Baron of Roslin and Jean, daughter of Henry Spotswood, Sheriff of Dublin.

His elder brother, James St Clair, had been killed fighting at the Battle of the Boyne in 1690 and therefore Alexander succeeded to the estates of Roslin.

[1] Alexander's poems are preserved in manuscript form in the Advocates Library.

[1] Alexander married Jean, daughter of Robert, 7th Lord Sempill.