Rhydderch ap Iestyn (died 1033) was king of Gwent and Morgannwg in south Wales and later took over the kingdom of Deheubarth and controlled Powys.
Comparatively little has been recorded about Rhydderch ab Iestyn in the annals. He appears to have originally been ruler of Gwent and Morgannwg, where his son later continued to have his power base. When Llywelyn ap Seisyll, king of Gwynedd and Deheubarth died unexpectedly in 1023, Rhydderch was able to seize Deheubarth, apparently by force of arms. In 1033 Rhydderch is recorded by Brut y Tywysogion as having been slain by the Irish, but with no explanation of the circumstances.
The kingdom of Deheubarth returned to the original dynasty in the form of Hywel ab Edwin and his brother Maredudd. A battle between Hywel and his brother and the sons of Rhydderch is recorded the following year. In 1045 Rhydderch's son, Gruffydd ap Rhydderch was able to seize Deheubarth from Gruffydd ap Llywelyn and held it for ten years until Gruffydd regained it.