Decades ago, {gamma}-ray observatories identified diffuse Galactic emission at 1.809MeV originating from {beta}^+^ decays of an isotope of aluminium, ^26^Al, that has a mean lifetime of 1.04 million years. Objects responsible for the production of this radioactive isotope have never been directly identified owing to insufficient angular resolutions and sensitivities of the {gamma}-ray observatories. Here, we report observations of millimetre-wave rotational lines of the isotopologue of aluminium monofluoride that contains the radioactive isotope (^26^AlF). The emission is observed towards CK Vul, which is thought to be a remnant of a stellar merger. Our constraints on the production of ^26^Al, combined with the estimates on the merger rate, make it unlikely that objects similar to CK Vul are major producers of Galactic ^26^Al. However, the observation may be a stepping stone for unambiguous identification of other Galactic sources of ^26^Al. Moreover, a high content of ^26^Al in the remnant indicates that, before the merger, the CK Vul system contained at least one solar-mass star that evolved to the red giant branch.