Rhydderch ap Iestyn (died 1033) was king of Deheubarth and controlled Powys.
Comparatively little has been recorded about Rhydderch ab Iestyn in the annals.
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.