The collective behavior of a many-body system near a continuous phase transition is insensitive to the details of its microscopic physics.
Characteristic features near the phase transition, called critical phenomena, are that the thermodynamic observables follow generalized scaling laws. The Berezinskii-Kosterlitz-Thouless (BKT) phase transition in two-dimensional Bose gases presents a particularly interesting case because the marginal dimensionality and intrinsic scaling symmetry result in a broad fluctuation regime which manifests itself in an extended range of scale invariant and universal behavior. Studies on BKT transition in cold atoms have stimulated great interest in recent years, clear demonstration of a universal behavior near the phase transition, however, has remained an elusive goal. Here we report the observation of a global invariance of scale and a universal description of 2D gases based on direct in situ optical imaging. The extracted universal thermodynamic functions confirm the wide critical regime near the BKT phase transition, provide a sensitive test to Monte Carlo calculations, and point toward a growing density-density correlations in the critical regime. Our assay raises new perspectives to explore further universal phenomena in the realm of classical and quantum critical physics.