The HP-35 was Hewlett-Packard's first pocket calculator and the world's first scientific pocket calculator (a calculator with trigonometric and exponential functions). Like some of HP's desktop calculators it used reverse Polish notation. Introduced at US$395, the HP-35 was available from 1972 to 1975.

Market studies had shown no market for pocket sized calculators at the anticipated price. In about 1970 HP co-founder Bill Hewlett challenged his co-workers to create a "shirt-pocket sized HP-9100". About twelve HP-35 portable calculators were made as a "hack" by and for other engineers at HP.^{[citation needed]} It is rumored^{[citation needed]} that the development engineer got carried away and included a full suite of scientific functions to satisfy requests from his co-workers. When these prototypes proved popular, HP decided to turn the HP-35 into a commercial product. The HP-35 was the first calculator with a full suite of trigonometric and transcendental functions.

In the first months orders were exceeding HP's expectations as to the entire market size, which was 10,000 units per year. Before the HP-35, the only practical portable devices for performing trigonometric and exponential functions were slide rules. Existing pocket calculators at the time were only four-function, i.e., they could only do addition, subtraction, multiplication, and division. It had been originally known simply as "The Calculator", but Hewlett suggested that it be called the HP-35 because it had 35 keys.