Ahmed was reported for the second time under the revised ICC bowling review regulations by the umpires during the first Test against England in Multan in November. His team-mate Shoaib Malik has been cleared.
The ban takes some of the shine off Pakistan's Test series victory over England.
Ahmed was first reported and suspended earlier this year but returned to international cricket following remedial work on his action and a full biomechanical analysis.
Having been reported and assessed as bowling illegally for a second time within two years of the first period of suspension, Ahmed has received a mandatory one-year ban.