The multidimensional character of competence development involves a profound methodological change in how the teaching process is conceived. An integrated STEAM education is one potentially useful methodological approach to assist in this matter. Using design research, an integrated STEAM approach was designed, implemented and assessed with 121 students in their sixth year of primary education. Its effect was analysed through a mixed methods study. The results show that the students obtain a high level of competence after the approach is applied in all of the key competences, with an emphasis on the level attained by girls in scientific skills. These results seem to reinforce the hypothesis that an integrated STEAM education is effective as a possible avenue for improving competence development in the primary education years as it didactically responds to the complexity of today's world. [Translation from Spanish by Jennifer Martin.]