The article analyses the possibilities of developing an integrated indicator and a model of the assessment of forests naturalness using the data from the database of mountainous spruce forests situated in the Western Carpathians of Slovakia. The article presents two variants of such a model, one based on discriminant analysis, while the second one using an additive approach. The analysis of the data from mountainous spruce forests revealed significant indicators of forest naturalness degree: the arithmetic mean of the ratio between crown length and tree height, the deadwood volume, the coverage of grasses, the coverage of mosses and lichens, and the aggregation index. In addition, the coefficient of variation of tree diameters was included in the final model, since its presence in the model had a positive influence on the correctness of the classification of the forest naturalness degree. The correctness of the classification of the proposed discriminant model was 74.5%. For the additive model, the ranges of the values of the integrated indicator were defined for every degree of forest naturalness by taking into account the error ranges of the arithmetic mean values and the percentiles of the values in individual degrees of forest naturalness. The overall correctness of the classification with the additive model was 63.4%. In the second step, the scheme how to apply the classification model of the forest naturalness degree in the decision-making process of designating as a forest protected areas was proposed. In this scheme, the degree of forest naturalness is considered as a basic criterion for the determination of nature-conservation value of forest ecosystems. As further decision-making criteria we identified the possibility to restore, or the possibility to improve the naturalness of less natural forest ecosystems, which are designated as protected; the occurrence of the endangered species; and the occurrence of other natural values.