Cardiff South and Penarth (Welsh: De Caerdydd a Phenarth) is a constituency of the Senedd.

It elects one Member of the Senedd by the first past the post electoral system.

The only major boundary changes that have occurred in recent years is the inclusion of the ward of Sully into the constituency for the 2007 National Assembly for Wales election.

The first vote is for a candidate to become the Member of the Senedd for the voter's constituency, elected under the first past the post system.

Additional member's seats are allocated from the lists by the d'Hondt method, to create a more proportional outcome in both the region and the Senedd.