The standard aerodynamic design of a centrifugal compressor features an unshrouded impeller with a smooth shroud. However, the shape of the shroud significantly influences the final compressor parameters. Previous research has shown that even small geometric changes in the shroud, referred to as casing treatment, can substantially affect the operating characteristics of a rotating machine. This article presents a novel casing treatment for a centrifugal compressor, involving the creation of ten circumferential grooves with a semi-circular profile in a meridional section. The radius of these grooves is approximately equal to the tip clearance. Experimental results demonstrate that the presence of these grooves shifts the surge line toward lower flowrates, increases efficiency in a narrow region between the best efficiency point and the surge line and shifts the choke line toward lower flowrates. External sound pressure measurements indicate a reduction in amplitudes at sub-synchronous frequencies, suggesting a potential link between the compressor map's influence and the suppression of aerodynamic instabilities. Computational fluid dynamics analysis supports the experimental observations, revealing a significant influence of energy dissipation caused by the applied grooves. This study highlights the considerable impact of shroud geometry on centrifugal compressors with unshrouded impellers and offers valuable insights for further research into casing treatments.