The 27-year-old has made 18 appearances for the Spireites this term, 12 of them starts, and has found the net three times, most recently at the start of this month in a 6-1 FA Cup demolition of Hartlepool, Walsall's opponents on Saturday against whom Westcarr may make his debut.
He signed for Chesterfield last summer, scoring 12 goals in 48 appearances as the Spireites failed to maintain their League One status.
Saddlers boss Dean Smith said: "Craig is a player who we have been aware of for some time. We had no hesitation in bringing him in when the opportunity presented itself to us.
"He played in League One last season and caused us problems, so we know that he can be a handful.
"He's quick, can carry the ball and link up the play and also score goals. He's also got that bit of experience which we need."