Mehsud was deputy leader of the banned Tehrik-e Taliban Pakistan (TTP), the main faction of the hardline Islamist Pakistani Taliban.
He was killed in a drone strike on Thursday in North Waziristan, near the border with Afghanistan, the militant group added in a statement.
The TTP has been blamed for dozens of suicide bombings and other attacks.
In December, gunmen from the militant group stormed a college in the Pakistani city of Peshawar, killing at least nine people and injuring 36.
Mehsud's death would further weaken the Pakistani Taliban, who have been forced to reduce their activities due to a relatively successful Pakistan military offensive, BBC Urdu's Haroon Rashid reports.
Pakistani officials claim many TTP militants have been forced to take refuge in Afghanistan as a result, our correspondent adds.
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