The kilometre per hour (American English: kilometer per hour) is a unit of both speed (scalar) and velocity (vector). The unit symbol is km/h or km·h −1 ; however, the colloquial abbreviations "kph" and "kmph" are sometimes also used in English-speaking countries, in analogy to mph, although these are not in accordance with international scientific standards. In North American slang and military usage, km/h (as well as km) is commonly pronounced, and sometimes even written, as klicks or kays (K's), although strictly these refer to kilometres rather than kilometres per hour.

The km/h is the worldwide most commonly used speed unit on road signs and car speedometers. Therefore, it is far more common in everyday usage than the proper SI unit of speed, m/s. Along with the kW·h, km/h is the most commonly used metric unit based on the hour, which is only a "non-SI unit accepted for use with the International System of Units" by the BIPM.