"An Open Letter to Hobbyists" is a 1976 open letter written by Bill Gates, the co-founder of Microsoft, to early personal computer hobbyists, in which Gates expresses dismay at the rampant software piracy taking place in the hobbyist community, particularly with regard to his company's software.

In the letter, Gates expressed frustration with most computer hobbyists who were using his company's Altair BASIC software without having paid for it. He asserted that such widespread unauthorized copying in effect discouraged developers from investing time and money in creating high-quality software. He cited the unfairness of gaining the benefits of software authors' time, effort, and capital without paying them as a rationale for refusing to publish the machine code for his company's flagship product, thereby making it available to lower-income hobbyists who could have borrowed such program listings from their local library and entered the program into their hobby computer by data entry.