WASHINGTON—In nearly every speech, Bernie Sanders reminds voters that he doesn’t have a super PAC, doesn’t want money from Wall Street and rejects establishment politics.

Yet the Vermont senator has benefited from at least $1.5 million in backing from super PACs and from political groups that don’t have to fully disclose their donors, according to filings with the Federal Election Commission.

As a member of Congress, he regularly attended retreats with Wall Street lobbyists and other donors, and as a candidate for the Senate, he benefited from money that indirectly came from them. Last year, he directly accepted about $55,000 in Wall Street contributions for his Democratic presidential campaign, FEC filings show.

To be sure, former secretary of state Hillary Clinton has raised far more from Wall Street donors, taking in $2.9 million last year, and the super PACs helping her collected $14.3 million from financial firms, about 30% of their total haul, according to an analysis from the nonpartisan Center for Responsive Politics. She has helped raise money for her primary super PAC, while Mr. Sanders hasn’t raised funds for outside groups. Overall, Mrs. Clinton raised $112 million last year, compared with $75 million raised by Mr. Sanders.

Mrs. Clinton says outside contributions supporting her campaign don’t influence her judgment. Mr. Sanders says that, for him, such contributions don’t exist. “I’m the only candidate up here…who has no super PAC,” he said Thursday during a Democratic debate.