Downing came through the ranks at The Hawthorns, signing pro terms in 2009, he never made a first team appearance for the Baggies but did gain experience during loan spells with, Hereford United, Rotherham United, Shrewsbury Town and Barnet.
20-year-old Downing has signed a one-year-contract following a successful trial.
Walsall have also announced the signing of former Coventry Sphinx and Aston Villa striker Connor Taylor, as reported here on Saturday.