Robert E. Rubin, arguably the best-known financier of his generation and the recently retired Treasury Secretary, has taken a top position at Citigroup, the nation's largest financial services company.

Mr. Rubin, 61, a former top official of Goldman, Sachs & Company, said yesterday that he had joined Sanford I. Weill and John S. Reed, the chairmen and chief executives Citigroup, in what Mr. Reed described as a ''three-person office of the chairman'' that will oversee what has become the first true American financial conglomerate since the Depression.

The appointment came less than a week after the Clinton Administration and Congress agreed on a compromise bill that would overhaul the laws that regulate the financial industry, a measure that removes many of the restrictions preventing banks, securities firms and insurance companies from buying one another or engaging in one another's businesses. Both Mr. Rubin and Citigroup strongly supported the bill, which would greatly benefit the company. Mr. Rubin said he played a role in arranging the final compromise that will probably lead to the repeal of the so-called Glass-Steagall legislation. But he said that had nothing to do with his decision to join the company.

The appointment settles the speculation about what Mr. Rubin, who presided over the Clinton Administration's economic policy during one of America's longest economic booms, would do for an encore. Mr. Rubin also discussed jobs with American International Group, a leading life insurance company, and Warburg Pincus, a New York investment firm, before agreeing to join Citigroup, said people who were told about Mr. Rubin's negotiations. He also had discussions with Goldman, Sachs about serving on his old firm's board, people close to Mr. Rubin said.