The concept of competitiveness occupies a central place in the economic thinking of all companies, and such a fact is not surprising considering that competitiveness is one of the key prerequisites for the survival and market success of a company in the domestic or international market. At the company level, one of the central concepts upon which a company's competitiveness depends is the concept of productivity represented by the ratio of production quantity to the amount of inputs used, and one of the available tools containing guidelines and methodology for improving the business components of a company and consequently having a direct impact on productivity is the international standard ISO 9001:2015; Quality Management Systems. The research conducted within this dissertation aimed to investigate, on a population of medium-sized manufacturing companies in the Republic of Croatia, whether there is an influence of the implementation and maintenance of a quality management system according to the ISO 9001:2015 standard on their competitiveness. The research model was based on examining the relationships between three constructs: the key ISO 9001:2015 principles, the company's motivation for maintaining ISO 9001:2015, and company competitiveness, where the construct key ISO 9001:2015 principles was viewed through three sub-constructs, namely knowledge management, change management, and continuous improvement. The proposed model assumed the influence of knowledge management, change management and continuous improvement on company competitiveness, with the moderating effect of the company's motivation for maintaining a quality management system according to the ISO 9001:2015 standard. This motivation had two main manifestations: it could be predominantly internal (when the motives for maintaining the quality management system come from within the company, e.g., the desire to increase the efficiency of its processes, reduce costs, etc.), or predominantly external (when motives come from outside the company, e.g., customer requirements, the fact that competitors have an ISO 9001 certificate, etc.). A positive influence of knowledge management and change management in line with the ISO 9001:2015 standard on company competitiveness was confirmed, thereby confirming and expanding the partial findings of other authors. A direct dependence of continous improvement on company competitiveness was not confirmed, but the existence of critical success points for the methodology of continuous improvement was confirmed, i.e., knowledge management and change management, which have a mediating role in the relationship between continuous improvement and competitiveness. Although the statistically significant influence of continuous improvement on competitiveness was not confirmed, a statistically significant positive influence of continuous improvement on knowledge management and change management was confirmed, which then consequently positively influences company competitiveness. It was also confirmed that the type of motivation for maintaining a quality management system according to ISO 9001:2015 standard has a moderating effect on the relationship between knowledge management and company competitiveness and between change management and competitiveness, but it does not affect the relationship between continuous improvement and competitiveness. In conclusion, based on the results of the conducted research, it can be said that by confirming the two main hypotheses and two auxiliary hypotheses, it was confirmed that Croatian companies from the processing sector, by maintaining a quality management system according to the ISO 9001:2015 standard, achieve a positive impact of the key principles of change management and knowledge management on their overall competitiveness, where the motivation for maintaining the quality management system has a crucial moderating effect.