Co-workers and relatives of Afridi confirmed the lawyer had left Pakistan, fearing for his life.
Afridi was defending Dr. Shakil Afridi, the man whose vaccination program in the city of Abbottabad helped find bin Laden, who was then killed by U.S. special forces in May 2011.
Dr. Afridi is facing murder charges related to the death of a patient eight years ago.
The lawyer's associates and relatives, all of whom requested anonymity, said militants had threated to kill attorney Afridi and his family.
Samiullah Afridi is the second member of the doctor's legal team to flee Pakistan.