Treatments involving vegetation thinning have become more frequent in Mediterranean forest ecosystems. Under extreme climate conditions, this practice can affect the balance between physicochemical, microbiological and biochemical soil properties, modifying the soil quality and the ability of these forest ecosystems to regenerate. The aim of this study was to use principal component analysis (PCA) to develop a multivariable soil‐quality index (SQI) applicable to the Mediterranean forest ecosystems of the Cuenca mountain range in central‐eastern Spain. To achieve this, seven physicochemical properties (moisture, pH, electrical conductivity, carbonate, total organic carbon, total nitrogen and available phosphorus) and six microbiological properties (basal respiration and microbial biomass carbon, as well as the activity of dehydrogenase, urease, phosphatase and β‐glucosidase enzymes) were measured seasonally between 2011 and 2012. Soil samples were obtained from seven unmanaged and undisturbed forest areas and these data were used to create the SQI. Subsequently, we applied this SQI to soil samples from other forests in the Cuenca mountain range, which had been affected by different thinning treatments. Our analysis showed that the greater was the canopy present in these treated forests, the larger were the average values for all 13 variables. The smallest SQI values were in areas that had been intensively thinned. Therefore, our results suggest that this SQI is an important tool for the development of appropriate measures to maintain the quality of forest soil in Spanish Mediterranean forest ecosystems. Highlights We developed a multivariable soil quality index (SQI) to assess status of Mediterranean forest ecosystems. This index is adequate to detect changes quickly in forest soil of Mediterranean ecosystems. The SQI helped to assess the effects of thinning treatments on forest soil. The SQI enabled establishment of different levels of soil quality (very high, high, medium and low).