The bird, seen behind Hamlyn Place, was first noticed by David Clegg at around 10am this morning.
Only one in a million magpies are reported to be white.
The unusual colouring is due to leucism, a genetic mutation that prevents pigment from developing in the feathers.
The Welsh Ornithological Society said: “White magpies are a very unusual sight.”
Leucism can also weaken feather structure, making flight more difficult.
The birds’ pale appearance makes them more visible to predators, and there is evidence they may be rejected by mates because of their colouring.
The British Trust for Ornithology asks those who spot white magpies to get in touch to help them understand the impact of the condition.