Despite the advocated importance of twenty-first century competences in education, it appears that they are not systematically integrated in national curricula, school curricula and classroom activities. Potential explanations can be found in the current ambiguity of definitions for twenty-first century competences and the absence of educational actors in curriculum development. Responding to both shortcomings, this study aims to explore how teachers' conceptualization and enactment of twenty-first century competences constitute dimensions that can inform curricular guidelines. An exploratory factor analysis of teachers' self-reported classroom activities that foster students' twenty-first century competences was conducted. The data comprised responses of 2,804 primary- and secondary-school teachers to a web-survey about the perceived occurrence of specific classroom activities targeting twenty-first century competences. Results suggested six coherent dimensions of classroom activities that foster twenty-first century competences: digital literacy, innovative thinking, critical thinking and communication, (digital) citizenship, self-regulated learning, and (computer-supported) collaborative learning. Nevertheless, there were substantial interrelationships among the six dimensions. This demonstrated that teachers did not perceive twenty-first century competences to be fostered by disconnected classroom activities, but that they already have a more integrative conception of the curricular innovation. The implications of these findings for curricula, teaching practice and research are discussed.