STEM (Science, Technology, Engineering and Mathematics) competences education is one of the topics of special relevance to present-day society, where research is carried out in this area from the perspective of didactics of science. A number of international institutions point to the need to introduce STEM education from an early age, with the aim of generating a positive effect on children's interest in choosing STEM careers. This research has focused on diagnosing the level of STEM competences that students acquire during their time in Primary Education, thus verifying the achievement of scientific and technological literacy at the end of this educational stage as a step prior to Secondary Education. The sample participating in this study was selected by non-probabilistic sampling due to ease of access. Specifically, 333 students from 5 schools in our region participated. The participating students were aged between 9-12 years old at the 4th, 5th and 6th grades of primary school. A diagnostic test consisting of 13 questions related to STEM content was designed as a measuring instrument. Some of the topics selected within the area of natural sciences were “Machines”, “Matter and Materials”, “Forces” or “Energy”. The results obtained show that there are important shortcomings in relation to STEM competences during the different courses that took part in the research. The descriptive statistical analysis of the data, as well as the inferential analysis, allowed us to establish comparisons among the different participating subjects, finding statistically significant differences (Sig. < 0.05) in the scores obtained. In general terms, the results seem to indicate that the contact that students have with STEM contents during their time in Primary Education is not completely significant, since most of the students who took the questionnaire were unable to pass it, even though they had recently dealt with the blocks of contents that appeared in it. This leads us to believe that students enter Secondary Education with scientific-technological deficiencies. We therefore consider it relevant to implement classroom methodologies that enhance the acquisition of STEM competences and allow students to work in a practical way with these contents, in order to promote a meaningful learning that lasts over time. In future research, it would also be interesting to study not only students but also teachers in order to find the starting point for this educational problem.