The Saddlers scored all their goals in a one sided first half, and although the relegation-threatened Seagulls improved later the result was never in doubt.
Walsall took the lead after 14 minutes when Wales under-21 international midfielder Mark Bradley celebrated his recall by driving in a low cross from the outstanding Alex Nicholls.
Brighton had little to offer going forward and Troy Deeney added Walsall's second with his 11th of the season in the 30th minute, from another Nicholls' cross.
Brighton had beaten Yeovil 5-0 on Saturday but looked a shadow of that team and any hopes they had died on 39 minutes when Walsall defender Anthony Gerrard headed the third after Bradley had hit the bar.
Brighton had plenty of the play in the second half but could not break down the home defence and their 379 fans who made the journey only had a shot from Dean Cox to cheer in the 84th minute but the effort was saved by Clayton Ince.