The 27-year-old, who joined the Spireites from Notts County in 2011, recently completed a loan spell with League One club Walsall.
Westcarr's time with Chesterfield will perhaps be best remembered for his goal in the 2-0 victory over Swindon in last season's Johnstone's Paint Trophy final.
Chief executive Chris Turner said: "Craig has been a good professional during his time with us and he'll always be remembered for his goal at Wembley.
"We wish him well for the future."
