In the cores of some clusters of galaxies the hot intracluster plasma is dense enough that it should cool radiatively in the cluster’s lifetime1,2,3, leading to continuous ‘cooling flows’ of gas sinking towards the cluster centre, yet no such cooling flow has been observed. The low observed star-formation rates4,5 and cool gas masses6 for these ‘cool-core’ clusters suggest that much of the cooling must be offset by feedback to prevent the formation of a runaway cooling flow7,8,9,10. Here we report X-ray, optical and infrared observations of the galaxy cluster SPT-CLJ2344-4243 (ref. 11) at redshift z = 0.596. These observations reveal an exceptionally luminous (8.2 × 1045 erg s−1) galaxy cluster that hosts an extremely strong cooling flow (around 3,820 solar masses a year). Further, the central galaxy in this cluster appears to be experiencing a massive starburst (formation of around 740 solar masses a year), which suggests that the feedback source responsible for preventing runaway cooling in nearby cool-core clusters may not yet be fully established in SPT-CLJ2344-4243. This large star-formation rate implies that a significant fraction of the stars in the central galaxy of this cluster may form through accretion of the intracluster medium, rather than (as is currently thought) assembling entirely via mergers.