PaineWebber & Co. was an American investment bank and stock brokerage firm that was acquired by the Swiss bank UBS in 2000. The company was founded in 1880 in Boston, Massachusetts, by William A. Paine and Wallace G. Webber. Operating with two employees, they leased premises at 48 Congress Street in May 1881. The company was renamed Paine, Webber & Co. when Charles Hamilton Paine became a partner. Members of the Boston Stock Exchange, in 1890 the company acquired a seat on the New York Stock Exchange. Wallace G. Webber retired after the business weathered a major financial crisis of 1893.

In May 1881, William Alfred (W.A.) Paine (with a loan from his father) and Wallace G. Webber founded Paine & Webber as a brokerage firm in Boston, Massachusetts with a seat on the Boston Stock Exchange. With the admission of Charles H. Paine to the partnership, the firm was renamed Paine, Webber & Co. The firm would purchase a seat on the New York Stock Exchange in 1890. Also in the 1890s, W.A. Paine entered into a partnership with Copper Range Company and Copper Range Railroad, controlled by John Stanton.

Controlled by the Paine family, Paine, Webber & Co. entered the investment banking business in the 1920s. After nearly fifty years at the head of the company, W.A. Paine died right before the Wall Street crash of 1929. His son F. Ward Paine became head of the firm, a position he held until 1940.

Following the difficult years of the Great Depression, Paine Webber merged with Jackson & Curtis, another Boston-based brokerage firm, in June 1942. In July 1879, Charles Cabot Jackson and Laurence Curtis had founded their brokerage firm Jackson & Curtis on Congress Street in Boston, not far from the original Paine Webber offices. The combined firm, Paine, Webber, Jackson & Curtis, operated a combined total of 22 branch offices. With its greater combined asset base, Paine Webber Jackson & Curtis had become a significant participant in the New England financial market.

In October 1960, Paine Webber managed the initial public offering (IPO) of the Green Shoe Manufacturing Co., in which it introduced the concept of stabilization covered by an overallotment option, which has ever since been known by the colloquial name of greenshoe.

The firm moved its headquarters from Boston to New York in 1963. The firm's holding company was incorporated on June 30, 1969, as PaineWebber Inc., of which Paine Webber Jackson & Curtis was its main subsidiary.

In 1974, the firm completed an initial public offering of the stock of its holding company, PaineWebber Inc., and listed the company on the New York Stock Exchange. PaineWebber engaged in a number of acquisitions in the 1970s, as a wave of consolidation spread through the industry. In 1973, the firm acquired F.S. Smithers & Co., providing its first a presence in fixed income. PaineWebber also acquired Abbott, Proctor & Paine in 1970, the Abacus Fund, a closed-end investment company in 1972, Mitchum, Jones & Templeton Inc. in 1973.

In 1977, the firm acquired investment research and advisory firm Mitchell Hutchins, which had been founded in 1919.