The Federal Aviation Administration (FAA) requires small unmanned aircraft systems (sUAS) to be registered with the agency if the device will be used outdoors and weighs between 0.55 lbs and 55 lbs.
The FAA 's Drone Zone offers tools for sUAS registration, either under Part 107 guidelines or under the exception for recreational flyers. Go to the Drone Zone »
UAS weighing over 55 pounds require paper (N-number) registration. N-number registration may also be required to operate small unmanned aircraft in some foreign jurisdictions. Learn more on the FAA website »
A video from the FAA explains more about the registration process. The FAA also has general registration information and UAS registration FAQs.