The warning, in place from 11am to 11pm, is due to Storm Boelo, which is expected to bring heavy rain and a high likelihood of moderate impacts from surface water flooding.
The Met Office has warned that 25-30mm of rain is expected widely across the warning area during this time, with a few places such as upland areas seeing 50-60mm, and some of this perhaps falling within six hours.
The rain is expected to ease for a time through the evening, but this will be followed by another intense band of heavy rain following in from the west, before the whole system clears away eastward to bring a clear night with a few scattered showers following on.
The Met Office has warned that spray and flooding on roads will probably make journey times longer, and bus and train services will probably be affected with journey times taking longer.
Some interruption to power supplies and other services is likely, and flooding of a few homes and businesses is possible.
The Met Office has advised people to check road conditions if driving, or bus and train timetables, amending travel plans if necessary.
People have also been advised to prepare for power cuts by gathering torches and batteries, a mobile phone power pack, and other essential items.
People have also been advised to check if their property could be at risk of flooding, and if so, to consider preparing a flood plan and an emergency flood kit.
The warning covers all of Wales, including Blaenau Gwent, Caerphilly, Carmarthenshire, Ceredigion, , Monmouthshire,, Newport, Pembrokeshire, and the Vale of Glamorgan.
It comes one day after Storm Austen battered parts of the country.
There was disruption to road and rail services, as well as wide spread power cuts – some of which lasted for several hours.
