Mark Esper opposes activating the Anti-Riot Act to deploy troops to quell demonstrations nationwide.
The Esper said the military deployment plan for executive roles should only be used "in the most urgent and serious case".
To deploy troops in the United States for executive purposes, Trump needs to activate the Anti-Riot Act of 1807, the most recently used in 1992 to respond to riots in Los Angeles.
Trump this week said he could use the military to stamp out violent demonstrations in US cities but has yet to activate the law.
During the press conference, Esper vehemently criticized the actions of the Minneapolis city police.
Floyd's plea for justice took place in at least 140 U.S. cities, and some extremists took advantage of the protests to loot, smash shops, burn cars and buildings.