Reid said he wasn't sure if the tip was true or not.
Reid refused to say who the investor was, according to the Huffington Post.
Democrats have repeatedly criticized Romney for not releasing enough of his tax returns. They say Romney's hesitance to release less than two years' worth of returns indicates he's hiding something.
Romney, meanwhile, has denied that he's hiding anything.
"That trustee follows all U.S. laws. All the taxes are paid, as appropriate. All of them have been reported to the government. There's nothing hidden there," Romney said in a recent interview with Radio Iowa.