Mary Bruce (c. 1282 - 1323) was the younger sister of Robert the Bruce, King of Scots. During the First War of Scottish Independence, she was captured by the English and imprisoned in a cage at Roxburgh Castle for about four years. She was the daughter of Robert de Brus, 6th Lord of Annandale, and Marjorie, Countess of Carrick.
Along with the king's other female relatives (Christina Bruce, Marjorie Bruce, his wife Elizabeth de Burgh and supporter Isabella MacDuff), she was captured and betrayed to the English by the Earl of Ross. By order of King Edward I of England, she was then held prisoner in an iron or wooden cage exposed to the public view in Roxburgh Castle. Isabella MacDuff was imprisoned in a similar cage at Berwick Castle. They were both transferred to better conditions in 1310. This was not necessarily a humane move, it has been suggested that by this stage Bruce was gaining strength, the women were potentially valuable hostages, and the English did not want them dying of exposure.
She was eventually released in exchange for English noblemen captured at the Battle of Bannockburn.