Krypton-85 (⁸⁵Kr) is a radioisotope of krypton, distributed throughout the atmosphere and presently forming about 15 ppt of atmospheric krypton on average.

Krypton-85 has a half-life of 10.728 years and a maximum decay energy of 687 keV. It decays into stable rubidium-85. Its most common decay (99.57%) is by beta particle emission with a maximum energy of 687 keV and an average energy of 251 keV. The second most common decay (0.43%) is by beta particle emission (maximum energy of 173 keV) followed by gamma ray emission (energy of 514 keV). Other decay modes have very small probabilities and emit less energetic gamma rays. Krypton-85 is mostly synthetic, though it is produced naturally in trace quantities by cosmic ray spallation.

In terms of radiotoxicity, 440 Bq of ⁸⁵Kr is equivalent to 1 Bq of radon-222, without considering the rest of the radon decay chain.

Krypton-85 is produced in small quantities by the interaction of cosmic rays with stable krypton-84 in the atmosphere. Natural sources maintain an equilibrium inventory of about 0.09 PBq in the atmosphere.

As of 2009, the total amount in the atmosphere is estimated at 5500 PBq due to anthropogenic sources. At the end of the year 2000, it was estimated to be 4800 PBq, and in 1973, an estimated 1961 PBq (53 megacuries). The most important of these human sources is nuclear fuel reprocessing, as krypton-85 is one of the seven common medium-lived fission products. Nuclear fission produces about three atoms of krypton-85 for every 1000 fissions (i.e., it has a fission yield of 0.3%). Most or all of this krypton-85 is retained in the spent nuclear fuel rods; spent fuel on discharge from a reactor contains between 0.13–1.8 PBq/Mg of krypton-85. Some of this spent fuel is reprocessed. Current nuclear reprocessing releases the gaseous ⁸⁵Kr into the atmosphere when the spent fuel is dissolved. It would be possible in principle to capture and store this krypton gas as nuclear waste or for use. The cumulative global amount of krypton-85 released from reprocessing activity has been estimated as 10,600 PBq as of 2000. The global inventory noted above is smaller than this amount due to radioactive decay; a smaller fraction is dissolved into the deep oceans.

Other man-made sources are small contributors to the total. Atmospheric nuclear weapons tests released an estimated 111–185 PBq. The 1979 accident at the Three Mile Island nuclear power plant released about 1.6 PBq (43 kCi). The Chernobyl accident released about 35 PBq, and the Fukushima Daiichi accident released an estimated 44–84 PBq.

The average atmospheric concentration of krypton-85 was approximately 0.6 Bq/m³ in 1976, and has increased to approximately 1.3 Bq/m³ as of 2005. These are approximate global average values; concentrations are higher locally around nuclear reprocessing facilities, and are generally higher in the northern hemisphere than in the southern hemisphere.

For wide-area atmospheric monitoring, krypton-85 is the best indicator for clandestine plutonium separations.