The highly-rated Defoe has already missed seven games this season because of his previous offences and his latest transgression in only the seventh minute of the game will prove extremely costly for the Hammers.
Defoe appeared to lash out at Ian Roper and referee Scott Mathieson had no hesitation in brandishing a straight red card.
Despite playing with only ten players West Ham gave themselves an immediate lift when Martin Harewood put the Hammers ahead just three minutes after the Defoesetback.
Paul Merson was often industrious but the former Arsenal star was badly at fault when he lost possession to enable the talented Matthew Etherington to provide the cross from which Harewood scored his third goal since his £500,000 move from Nottingham Forest.
Merson was also badly at fault early in the second half when a bad mistake by Ian Pearce put the Walsall man in possession but he hastily volleyed the ball high over the bar.
With David James in commanding form to frustrate his former Villa colleague, Merson, a few minutes later it appeared as if ten-men West Ham would hold out for a win to push them into the play-off zone.
But in the 69th minute a relieved Walsall side finally managed to equalise. A cross from star-man Vinny Samways, who was back in favour after missing the previous home game, was headed down by Darren Wrack for Jorge Leitao to score from close range - his fourth strike of the season.