The quadrupole mass spectrometer of the Sample Analysis at Mars (SAM) instrument on Curiosity rover has made the first high‐precision measurement of the nonradiogenic argon isotope ratio in the atmosphere of Mars. The resulting value of ³⁶Ar/³⁸Ar = 4.2 ± 0.1 is highly significant for it provides excellent evidence that “Mars” meteorites are indeed of Martian origin, and it points to a significant loss of argon of at least 50% and perhaps as high as 85–95% from the atmosphere of Mars in the past 4 billion years. Taken together with the isotopic fractionations in N, C, H, and O measured by SAM, these results imply a substantial loss of atmosphere from Mars in the posthydrodynamic escape phase.