The röntgen or roentgen (symbol R) is a unit of exposure to ionising radiation (X or gamma rays), and is named after the physicist Wilhelm Röntgen. It is the amount of radiation required to liberate positive and negative charges of one electrostatic unit of charge in 1 cm³ of air at standard temperature and pressure (STP). This corresponds to the generation of approximately 2.08×10⁹ ion pairs.

In SI units, 1 R = 2.58×10^-4 C/kg.

Natural background exposure is around 10 μR per hour. A dose of 500 R in 5 h is lethal for humans.

The röntgen is an obsolete unit, superseded by the gray. In dry air, 1 Gy is approximately equal to 100 röntgens.

The röntgen was occasionally used to measure exposure to radiation in other forms than X-rays or gamma rays. To adjust for the different impact of different forms of radiation on biological matter, the "Röntgen equivalent man" or rem was also in use. Exposure in rems is equal to the exposure in röntgens multiplied by the Q value, a constant describing the type of radiation. The rem is now superseded by the sievert (see the latter for a list of Q values).