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 at the time had shown no market for pocket sized calculators. However, HP co-founder Bill Hewlett began development of a "shirt-pocket sized HP-9100 ", and it turned out that the marketing studies were wrong. In the first months orders were exceeding HP's expectations as to the entire market size. Before the HP-35, the only practically portable device for performing trigonometric and exponential functions were slide rules. Existing pocket calculators at the time were only four-function; i.e., 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.

The calculator used a traditional floating decimal display for numbers that could be displayed in that format, but automatically switched to scientific notation for other numbers. The fifteen digit LED display was capable of displaying a 10 digit mantissa plus its sign and a decimal point and a two digit exponent plus its sign. The display was unique in that the multiplexing was designed to illuminate a single LED segment at a time (rather than a single LED digit) because HP research had shown that this method was perceived by the human eye as brighter for equivalent power.

The calculator used 3 "AA"-sized NiCd batteries in a battery pack that could be exchanged for extended run time. (An external battery charger was available and the calculator could also run from the charger.)

Internally, the calculator was organized around a serial (one-bit) processor chipset processing 56-bit (???) floating-point numbers.

The HP-35 was the start of a family of related calculators which all shared similar mechanical packaging:

• The HP-45 added many more features including the ability to control the output format (rather than the purely automatic format of the HP-35). It also contained a hidden timer feature.
• The HP-65 added programmability with program storage on magnetic cards.
• The HP-55 provided storage for smaller programs (but didn't provide any storage). The hidden timer that was present on the HP-45 was now crystal-controlled and explicitly documented.
• The HP-72 (???) provided financial rather than scientific functions such as future value and net present value.