The left-arm quick replaces paceman Nathan Bracken after the Australian told the Midlands-based county that a revival in his international fortunes meant he might not be available for most of the English season, which starts next month.
Zaheer, 27, made his name on the international stage during India's 2002 tour of the West Indies and was a key member of the team that a year later lost to Australia in the World Cup Final.
An automatic selection before a hamstring injury interrupted his career, the likes of Munaf Patel and Shanthakumaran Sreesanth are currently denying him a place in India's pace attack.
And that has left Worcestershire confident that, subject to clearance from the Board of Control for Cricket in India (BCCI), Zaheer will be available for the entire English season following his expected arrival in mid-April.
In a Worcestershire statement Zaheer, who has played 42 Test Matches and 107 ODIs for his country, taking 121 and 155 wickets respectively, said: "To have the opportunity to play for Worcestershire is a privilege.
"I very much look forward to the season ahead and to re-establishing myself on the international scene whilst helping the county to win as many matches as possible.
Worcestershire's director of cricket, Steve Rhodes, said: "Were pleased for Nathan that he has had such a successful six months since we signed him but we really want stability with our overseas players this year.
"We targeted a left arm fast bowler from the start of our deliberations for 2006. Zaheer has had an outstanding domestic season in India and is exactly the type of player we feel will bring variation to our bowling attack," the former Worcestershire and England wicket-keeper added.