Isabel Marshal (9 October 1200 – 17 January 1240) was a medieval English countess.

The marriage was an extremely happy one, despite the age difference, and the couple had six children: Isabel's husband Gilbert joined in an expedition to Brittany in 1229, but died 25 October 1230 on his way back to Penrose, in that duchy.

His body was conveyed home by way of Plymouth and Cranborne, to Tewkesbury, where he was buried at the abbey.

These were most likely the reasons for both the proposal of marriage from Richard, 1st Earl of Cornwall, and Isabel's acceptance of it, despite the fact that her husband had been dead for only five months.

Isabel died of liver failure, contracted while in childbirth, on 17 January 1240, at Berkhamsted Castle.