Self organisation provides an elegant explanation for how complex structures emerge and persist throughout nature. Surprisingly often, these structures exhibit remarkably similar scale-invariant properties. While this is sometimes captured by simple models, the connection to real-world systems is exceptionally hard to test quantitatively. Here we identify three key signatures of selforganised criticality in the dynamics of a driven-dissipative gas of ultracold atoms and provide a first characterisation of its universal properties. We show that population decay drives the system to a stationary state that is largely independent of the initial conditions and exhibits scale invariance and a strong response to perturbations. This establishes a well-controlled platform for investigating self-organisation phenomena and non-equilibrium universality with unprecedented experimental access to the underlying microscopic details of the system. Self-organised criticality (SOC) is a fascinating concept, first put forward by Bak, Tang and Wiesenfeld in 1987 as a way to explain the large abundance of scale-invariant systems found in nature [1]. It is thought to underlie a wide range of complex dynamical phenomena, ranging from activity in electrical circuits and neural networks [2, 3], to the likelihood of avalanches and earthquakes [4] as well as how forest fires [5, 6], diseases [7] and even ideas spread [8]. However, despite the wide ranging fundamental and practical importance of 1 arXiv:1806.09931v2 [cond-mat.quant-gas]