This correlational study addressed two issues pertinent to developing mathematical competences for the 21st Century. Firstly, in a post-digital society, technology is recognized in teachers' pedagogic and subject-specific knowledge domains. Secondly, cross-curricular teaching must be introduced to respond to societal requirements for maths knowledge application in diverse areas of life and work. Croatian educational reform recognized the importance of informatics and the integration of ICT across curricula supporting cross-curricular teaching. When reforming teaching, teaching efficacy beliefs inform change, and the study examined how technology pedagogy content knowledge (TPACK) contributes to teaching efficacy beliefs. Six hundred and six Croatian elementary school classroom teachers were surveyed for maths teaching efficacy beliefs (MTEBI) and self-assessment of their technology pedagogy content knowledge (TPACK). The study confirmed that the integration of technology knowledge in teachers' pedagogical knowledge is, in addition to content knowledge, essential and predicts, with a large effect size (f[superscript 2] = 0.64), the MTEBI subscale math teaching efficacy (MTE), while isolated technology knowledge does not. In addition, there is a positive correlation between years of service and pedagogy content knowledge and a negative correlation between years of service and technology pedagogy knowledge and technology knowledge, however, with a small effect size.