Ermessenda de Castellbò (died 1237), was an Andorran feudal ruler.
She was suo jure ruling Viscountess of Castellbò in Andorra 1226-1237.
In 1208, she married Roger-Bernard II, Count of Foix.
Through her, Andorra and Foix was united in 1278.
Because of her Cathar faith, her and her father's remains were exhumed and burnt by Inquisitor Pere de Cadireta in 1267.