"Talk is cheap; the work you do is what will endure," Wray told the 35,000 FBI staff in an internal letter.
"You've all been through a lot in the past nine months and I know it's often been unsettling, to say the least. And the past few days haven't done much to calm those waters," he said.
"Let me be clear: I stand fully committed to our mission... I stand with you."
Trump wrote on Twitter ahead of the memo's release: "The top Leadership and Investigators of the FBI and the Justice Department have politicized the sacred investigative process in favor of Democrats and against Republicans - something which would have been unthinkable just a short time ago."
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The memo's release raised concerns that Wray could be forced out of the FBI just six months after being appointed by Trump, who fired Wray's predecessor James Comey last May.
He praised the agency's high integrity, work ethic and professionalism that is "unrivaled anywhere in the world."
"We speak through our work. One case at a time. One intelligence product at a time. One decision at a time," he wrote.
"We stay laser-focused on doing great work, even when it's not easy, because we believe in the FBI.