"I decapitated with blessed right hand the head of the American Jew, Daniel Pearl, in the city of Karachi, Pakistan," Mohammed said in a written declaration submitted to a military tribunal at Guantanamo last weekend.
The Pentagon says it initially redacted the confession so that it could notify Pearl's family before the admission became public.
ABC News had earlier reported that Mohammed, known by the initials KSM, was the person who decapitated Pearl in a video that was distributed worldwide.
"For those who would like to confirm, there are pictures of me on the Iternet holding his head," KSM said in his statement.
US officials had told ABC News that identifying marks on the hand of the masked man holding Pearl's head matched those of KSM.