Walter Stewart, Earl of Atholl, Strathearn and Caithness (c. 1360 - died 26 March 1437) was a Scottish nobleman, the son of Robert II of Scotland. Stewart was an enthusiastic advocate of the ransom and return to Scotland of the future king in exile, James I, in 1424. In 1425 he served as a member of the jury of 21 which tried and executed his nephew Murdoch Stewart, 2nd Duke of Albany. Eventually however Atholl turned against the King and conspired in his assassination in 1437. He was tried for murder and was executed after 3 days of gruesome torture.
He was a son of Robert II of Scotland by his second wife Euphemia de Ross, daughter of Aodh, Earl of Ross. He was also a younger half-brother of Robert III of Scotland and an uncle of the above-mentioned James I of Scotland.
He married first, sometime before 1378, Margaret Barclay, Lady of Brechin, by whom he had two sons:
In 1390, his niece Euphemia resigned to him the Earldom of Caithness. In 1404, he was created Earl of Atholl.
He was energetic in retrieving his nephew James I from the Kingdom of England, which was accomplished in 1424, and was a member of the jury which tried his cousin Murdoch Stewart, 2nd Duke of Albany, and which culminated in the execution of Albany and two of his sons. He was made Great Justiciar of Scotland and Earl of Strathearn, with such title being taken from Malise Graham, who subsequently became the Earl of Menteith in 1427. He resigned Caithness to his son Alan in 1428, but regained it on Alan's death without issue in 1431.