"So the word is out that he hasn't paid any taxes for 10 years. Let him prove that he has paid taxes, because he hasn't. We already know that from one partial tax return that he gave us, he has money hidden in Bermuda, the Cayman Islands and a Swiss bank account," Reid said.
Democrats and some in Romney's party have called on the presumptive GOP nominee to make more of his tax returns available to the public.
"He's refused to release his tax returns, as we know. If a person coming before this body wanted to be a Cabinet officer, he couldn't be if he had the same refusal Mitt Romney does about tax returns," said Reid on the floor.
Reid also commented on Romney's wealth, saying that the GOP hopeful "makes more money in single day than the average American middle-class family makes in two years."
The Senate leader's remarks came during a speech slamming Romney's tax proposal as favoring the wealthy at the expense of middle- and lower-income families.
Later Thursday, House Speaker John Boehner (R-Ohio) blasted Reid for his suggestion that Romney hadn't paid taxes, saying his Senate colleague was making statements "without any facts."
"It's one of the problems that occurs here in Washington, people run out there without any facts and just make noise. The American people are too smart for this, they'll get to the bottom of this, it clearly is not a fact, and I would think that the Senate majority leader would be smart enough to know that," Boehner said during an appearance on "Kilmeade and Friends."
Romney has released his tax return for 2010 and an estimate of his 2011 tax obligation. In an interview with NBC News last week, he maintained that he would not make any further tax records available.
"I’ve released all the information about my financial holdings that's required by law," Romney said. "And then in addition beyond the law, have released, or will finally release act — when the last year is complete, two years of full returns."
Read more on Boehner's reaction on The Hill's Blog Briefing Room.
—Justin Sink contributed.