The Met Office says isolated heavy, and possibly severe, thunderstorms are expected to develop on Friday evening across parts of Wales.
It says some torrential downpours are possible leading to localised surface water flooding.
Hailstorms and frequent lightning are also possible hazards.
It says the public should be aware that there is a chance of some very localised significant disruption.
The yellow 'be aware' warning is in force from 7pm to midnight.