John's precise date of death is unclear. He could have died in or after 1393 and before 1397, when his younger brother Denis, Lord of Cifuentes, received support from legitimists in Portugal (implying that his older brother, who would have had a better claim, was already deceased). The chronicler Fernão Lopes said that he was dead by 1397. On 19 September 1396, Henry III of Castile confirmed on John's daughters their right to succeed in their father's county of Valencia de Don Juan, suggesting that John may have been dead by then. The Portuguese genealogist Anselmo Braamcamp Freire cites a letter from John I of Portugal, dated 20 September 1400, in which he refers to his deceased namesake half-brother, but he also suggests that the date might be wrong due to statements that Duke John was active in 1402 in confirming privileges given by Henry III of Castile to Palencia Cathedral., citing John's estates were partitioned between his legitimate daughters on 2 November 1404 at Medina del Campo.
