A stock trader is an investor in the financial markets. : A stock trader is an investor in the financial markets. Stock traders can be individuals or professionals trading on behalf of a financial company. Stock traders participate in the financial markets in various ways. Institutional level traders typically have the greatest market influence and are often employed by management investment companies, portfolio managers, pension funds or hedge funds. Stock trading can be profitable for individuals and institutions. The two groups participate in the financial markets in different ways. (See also: Introduction to Stock Trader Types.)