The sole dissenting commissioner, Robert Jackson, an independent who fills one of the Democratic seats on the commission, said in an interview: “Does this rule require customers’ interest to come first? No, it doesn’t.”

Mr. Clayton, the agency chairman, took issue with that characterization.

The rules will require brokers to “act in the best interest of their retail customers when making a recommendation, including not placing their financial or other interests ahead of the interests of the retail customer,” he said.

It will take some time for experts to fully grasp the changes outlined, and even longer to see how they might alter the brokerage landscape in practice. The section on Regulation Best Interest alone is 771 pages, and lawyers, academics and consumer advocates are still parsing and digesting the language. Even so, industry officials were comfortable heralding the rules, which will take effect 60 days after they are published in the Federal Register.

Kenneth E. Bentsen Jr., the president and chief executive officer of the Securities Industry and Financial Markets Association, the trade group for large financial services firms, said the rule would impose a materially heightened standard of conduct for broker-dealers.

“The costs to implement will no doubt be significant,” he said, but they are “worthwhile to uniformly enhance investor protection to the level investors should and do expect.”

It can be hard for investors to figure out where the loyalties of professionals — particularly brokers — lie. That tripped up Heather Heckel, 33, an art teacher at a middle school in Port Washington, N.Y., who previously worked in Manhattan.

A broker left a notice in Ms. Heckel’s school mailbox about investments the broker was selling inside retirement plans known as 403(b)’s, which are similar to 401(k)’s. She signed on, and the broker transferred her retirement savings from a guaranteed investment that paid 7 percent — an exceedingly rare investment, available to New York City public schoolteachers — and moved it to an annuity with an annual fee of more than 2 percent. That was more than four times the fee that the average investor pays for mutual fund-type investments, according to Morningstar.