For U.S. citizens, traveling to and from Puerto Rico is no different than traveling to and from other U.S. states and territories. No U.S. passport is necessary for a U.S. citizen traveling to Puerto Rico, or returning from Puerto Rico. However, some form of identification, such as a driver's license, an identification card issued by a state or Federal agency or an original birth certificate, should be carried for identification purposes.
For immigration purposes, traveling from the continental United States to Puerto Rico does not constitute an exit from the U.S. as long as the flight goes directly from any state to Puerto Rico. Likewise, when coming to the U.S. mainland from Puerto Rico, travelers do not have to go through Immigration, as long as the flight comes directly from Puerto Rico to any state.
However, as in any other U.S. state or territory, immigration enforcement agencies have the authority to interview people to verify legal status. It is a legal requirement for aliens lawfully present in the U.S., including Puerto Rico, to be in possession of the appropriate visa or official documentation establishing their legal immigration status.