Sir Thomas Bruce, 1st Baron of Clackmannan (died before 1348[1]) was the first Baron of Clackmannan and the possible grandson or grandnephew of King Robert the Bruce. Most Bruces of today are descended from Thomas Bruce's lineage. and current Chief of the Clan, Andrew Bruce, 11th Earl of Elgin, is a descendant of Sir Thomas. Thomas' son, Robert, became the second Baron of Clackmannan.
Thomas Bruce was born a descendant of Robert de Brus, 1st Lord of Annandale and was a member of the Scottish Bruce family. Thomas was granted land in Clackmannan by his cousin Robert II of Scotland after organising a revolt against the English in 1334. He married Marjorie Charteris of Stenhouse and it is from this line which most Bruces descend. Their issue included Robert Bruce, 2nd Baron of Clackmannan.[1]