The relative simplicity of the lowest-lying charmonium states can be used as benchmarks to test the QCD and QCD-inspired models, as these states lay in the transition region between perturbative and non-perturbative QCD. While the properties and many decay channels of some charmonium states below the open-charm threshold (such as $h_c$ or $\eta_c(2S)$) are still far from being known, many high-mass charmonium states above the open charm threshold predicted by potential models are not been found yet. Since 2009, BESIII has been scanning and investigating the charmonium spectrum to shed light on the internal structure of charmonium states. Thanks to its unique $J/\psi$, $\psi(2S)$ and even large XYZ data sets, BESIII could overcome statistical limitations. Recent results on charmonium decays from BESIII are presented.