In a letter to House Ways and Means Committee Chairman Richard Neal, Mnuchin said the committee's request lacked "legitimate legislative purpose" and therefore the Treasury Department was not constitutionally authorized to disclose the returns.
"Out of respect for the deadlines previously set by the committee and consistent with our commitment to a prompt response, I am informing you now that the Department may not lawfully fulfill the committee's request," Mnuchin wrote.
Mnuchin also reiterated his stance that the release of Trump's tax returns could threaten the privacy of all other tax payers.
"As you have recognized, the committee's request is unprecedented and it presents serious constitutional questions, the resolution of which may have lasting consequences for all taxpayers," he wrote.
Mnuchin's "final decision" on the release of Trump's returns came after he twice missed deadlines set by Neal and the committee.
Neal issued a short statement in response to Mnuchin's letter, saying he would "consult with counsel and determine the appropriate response."
The committee requested the release of Trump's business and personal returns for the past six years.
Neal directly requested the release of the records from IRS Commisioner Charles Retting, who deferred to Mnuchin's guidance on the final decision.