Ajmal is currently the highest-ranked Test spinner and will join Hampshire on a two-month deal as a replacement for overseas signing George Bailey.
Australia Twenty20 captain Bailey is due to be with his national team's limited-overs squad from August, and in his absence Hampshire will turn to Ajmal as they attempt to earn promotion from the LV= County Championship Second Division.
"I am very pleased to be joining Hampshire to play in the County Championship," Ajmal said in a statement.
"I have always enjoyed playing in England and look forward to playing my part in helping Hampshire get promoted."
Ajmal, 35, has taken 122 wickets in 23 Tests and played a starring role as Pakistan secured a series whitewash against England in the United Arab Emirates at the start of the year.
While the Pakistani is not a like-for-like replacement for Bailey, Hampshire face the prospect of losing young spinner Danny Briggs at the same time to England duty.
Hampshire manager Giles White said: "As things stand, it is possible that both George and Danny could be involved in the ODI series between England and Australia so the availability of Saeed for August and September is a great fit.
"He is one of the best bowlers in the world in all forms of the game and will add real quality to the squad at the business end of the season."
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