WASHINGTON—Scott Pruitt, the head of the Environmental Protection Agency, fought back Thursday against Democratic criticism of his spending and policies, asserting that the attacks were driven by opposition to President Donald Trump’s agenda.

In largely partisan House subcommittees hearings, Mr. Pruitt said he recognized the “very troubling reports” on his travel, housing and personnel actions, but that much of what had been reported was twisted. “Let me be clear, I have nothing to hide,” he said.

The EPA administrator is under investigation for his spending on office furnishings, personnel moves, travel costs and destinations, security practices and the link between his rental housing in Washington and a lobbyist. The White House, inspector general of the EPA, House oversight committee and Government Accountability Office have been looking at the issues.

Mr. Pruitt, who as the Oklahoma attorney general sued the Obama administration over its environmental policies, was tapped for the EPA post to lead the rollback of regulations Republicans deemed burdensome.

On Thursday, Mr. Pruitt said he reversed course on several of the issues now under investigation when he became fully aware of them or if they became a distraction.