Officers were investigating what the man was doing in the border area, which is believed to be a possible hiding place for Osama bin Laden and other foreign extremists, said one officer, Pir Shahab.
He said the man identified on his passport as Juddi Kenan did not have permission to be in the region as is required by Pakistani law. He was arrested at a checkpoint trying to enter Mohmand agency, Shahab said.
"He is holding an American passport, which shows him as a resident of Florida," Shahab said.
Another police official, Marjan Khan at the station in Sarrokali, said the man was wearing traditional Pakistani clothes and appeared to be a civilian. "He has told us that he was a student at a community college in Florida, and wanted to enter the tribal region to see a friend." Khan said the man carried a laptop and a travelling bag, adding that he had been shifted to an unknown place for more questioning, also by intelligence agencies.
A U.S. embassy spokesman said he had no information on the arrest.
Asked whether he was believed to be a journalist, a tourist, a researcher, or a suspected militant, Shahab said: "These are the questions we are trying to investigate."
Militants in the tribal regency are blamed for rising attacks in Pakistan and Afghanistan.
Pakistani authorities often claim to kill or arrest foreign extremists, mostly from Afghanistan, the Middle East and Chechnya, in the area.