Ajmal will join the Midlands County as their second overseas player for the domestic Twenty20 competition, along with Australia batsman Phil Hughes who will be at New Road for the whole of the 2012 English county season.
The in-form Ajmal took 24 wickets in Pakistan's 3-0 rout of England in the United Arab Emirates, including a Test-best seven for 55 in the series opener.
He spent the second half of last term with Worcestershire and helped them avoid relegation from the First Division of the County Championship.
Ajmal also collected up 16 Twenty20 wickets for the county at an average of just over 11 apiece.
"Ajmal showed last summer when playing for us what a quality bowler he is," said Worcestershire director of cricket Steve Rhodes.
"His recent performances against England only serve to back this up," the former England wicket-keeper added.
"We are delighted that he has agreed to return to New Road for a second time."
The 34-year-old Ajmal, in a Worcestershire statement, said: "I really enjoyed my time at New Road in 2011 and look forward to returning this summer to help inspire the club to Twenty20 success."
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