Isabella of Mar (c. 1277 - 1296) was the first wife of Robert Bruce.

She was the daughter of Donald, 6th Earl of Mar and a woman named Helen (or Ellen), who had previously been the wife of Malcolm Macduff, 7th Earl of Fife. An educated woman, Isabella spoke Gaelic and English. Her lands lay mostly to the northeast of Inverness.

Isabella was a wealthy young woman at the time of her arranged marriage to the Earl of Carrick, Robert Bruce. Her father was one of the seven guardians of Scotland who believed Robert Bruce to be the rightful King of Scotland. Despite the risks, he could see the advantage of the two families joining in marriage and bearing an heir to the throne. Mar was the first to sign over the estates of his family to the Bruce.

Isabella was married to Robert at the age of 18 and legend has it that they were much in love. Shortly after their marriage Isabella became pregnant. She had a healthy pregnancy but she died soon after giving birth to a daughter, Marjorie Bruce in 1296.

Robert married his second wife, Elizabeth de Burgh, 6 years later.

Isabella's daughter Princess Marjorie (d. 1316) married Walter Stewart, 6th High Steward of Scotland, and their son became Robert II of Scotland. From him descend the monarchs of the House of Stuart and the later royal families of Scotland and England.