Rhydderch's date of death is unknown, although the Life of Kentigern places his death in the same year as the saint's which, according to the Welsh Annals, occurred in 612, which is adjusted by historians to 614.
Descoñécese a data da morte de Rhydderch, aínda que a Vida de Kentigern sitúa a súa morte no mesmo ano cá do santo, que, segundo os anais galeses acaeceu no 612, data que os historiadores axustan a 614.