According to Article 2, Section 2, Clause 1 of the Constitution, the president "shall have power to grant reprieves and pardons for offenses against the United States except in cases of impeachment." According to the case of Ex parte Garland (1867), the scope of the president's pardon power is quite broad. And according to United States v. Klein (1871), Congress cannot limit the president's grant of an amnesty or pardon.
On August 25, 2017, Trump pardoned Joseph M. Arpaio, the longtime sheriff of Maricopa County, Arizona, for his conviction for criminal contempt of court on July 31, 2017. He had not yet been sentenced.
On March 9, 2018, Trump pardoned Kristian Mark Saucier, a former U.S. Navy sailor, for his conviction for unauthorized retention of national defense information on August 19, 2016. He was sentenced to 12 months' imprisonment and three years' supervised release, conditioned upon six months' home confinement and the performance of 100 hours' community service.