The only naturally occurring isotope of bismuth, 209Bi, is commonly regarded as the heaviest stable isotope. But like most other heavy nuclei abundant in nature and characterized by an exceptionally long lifetime, it is metastable with respect to α-decay. However, the decay usually evades observation because the nuclear structure of 209Bi gives rise to an extremely low decay probability and, moreover, generates low-energy α-particles difficult to detect. Indeed, dedicated experiments attempting to record the α-decay of 209Bi in nuclear emulsions failed. However, scintillating bolometers operated at temperatures below 100mK offer improved detection efficiency and sensitivity, whereas a broad palette of targets could be available. Here we report the successful use of this method for the unambiguous detection of 209Bi α-decay in bismuth germanate detectors cooled to 20mK. We measure an energy release of 3,137 +/- 1 (statistical) +/- 2 (systematic) keV and a half-life of (1.9 +/- 0.2) × 1019yr, which are in agreement with expected values.