William St Clair, Lord Herdmanston, Baron of Carfrae and Cessford, was a Scottish noble of the 14th century.
William was the son of William St Clair of Herdmanston, who had fought at the Battle of Bannockburn in 1314.
[1] William married Margaret, daughter of William St Clair, Master of Roslin (died 1330), they are known to have had the following issue: After the death of William, Margaret was remarried to Thomas Stewart, Earl of Angus and Chamberlain of Scotland, in 1357.