President Trump has reversed an Obama-era memorandum dictating how and when the U.S. government can deploy cyberweapons against its adversaries, in an effort to loosen restrictions on such operations, according to people familiar with the action.

Mr. Trump signed an order on Wednesday reversing the classified rules, known as Presidential Policy Directive 20, that had mapped out an elaborate interagency process that must be followed before U.S. use of cyberattacks, particularly those geared at foreign adversaries.

The...