In a letter to the House Committee on Ways and Means, Mnuchin repeated his rejection of the panel's request for the documents.
"In reliance on the advice of the Department of Justice, we have determined that the committee's request lacks a legitimate legislative purpose, and ... the department is therefore not authorized to disclose the requested returns and return information," Mnuchin said in the letter to committee Chairman Richard Neal.
The Democrats had hoped a nearly 100-year-old tax code provision requiring the Treasury to furnish tax returns upon request by Congress would get them the documents.
Mnuchin accused Democrats of "weaponizing" the IRS to get Trump's tax returns for political gain.
"First of all, we haven't made a decision, but I think you can guess which way we are leaning on our subpoena," Mnuchin said Wednesday. "To the extent that we don't and there is litigation, I take great comfort that there's a third branch of government to deal with this important issue."
While Congress could be helped by a New York bill where the body could request Trump's state tax returns, Neal's spokesman Dan Rubin said the committee would still want access to the federal data.
"Our request is in furtherance of the committee's investigation into the mandatory presidential audit program and to decide whether that program needs to be codified into federal law," Rubin said. "This is part of our oversight over the IRS. State returns from [New York] would not help that investigation."
Last week, the New York Senate approved a bill Wednesday giving state lawmakers access to Trump's state tax returns. Officials expect the State Assembly to pass the legislation as well.