Abstract The atmospheric abundance of 81 Kr is a global integrator of cosmic rays. It is insensitive to climate shifts, geographical variations, and short‐term solar cycle activity, making it an ideal standard to test models of cosmic‐ray flux on the time scale of 10 5 years. Here we present the first calculation of absolute 81 Kr production rates in the atmosphere, and a measurement of the atmospheric 81 Kr/Kr abundance via the Atom Trap Trace Analysis method. The measurement result significantly deviates from previously reported values. The agreement between measurement and model prediction supports the current understanding of the production mechanisms. Additionally, the calculated 81 Kr atmospheric inventory over the past 1.5 Myr provides a more accurate input function for radiokrypton dating. Plain Language Summary Krypton‐81 is a long‐lived radioactive isotope produced in the Earth's atmosphere by cosmic rays. It stays in the atmosphere as a noble gas for hundreds of thousands of years until its eventual nuclear decay. As a result, its abundance uniquely reflects the long‐term accumulation record of cosmic rays across the entire globe. We performed the first precise measurement of the atmospheric abundance of krypton‐81. The result agrees with the prediction of a realistic isotope production model, thus confirming the current understanding of the cosmic‐ray flux, isotope production mechanisms, and the past terrestrial and space magnetic field environment. Key Points Conducted new precision measurement of 81 Kr isotopic abundance in the atmosphere; new result differs significantly from previous values First ever cosmic‐ray flux models of 81 Kr production in the atmosphere are presented; models successfully predict new measured value Measurement and models provide more accurate input function for radiokrypton dating over the past 1.5 Myr