Abstract The study was conducted to explore the promotion of inclusive education in Nigerian secondary schools with a focus on 21st century Science, Technology, Engineering and Mathematics (STEM) teachers. Two research questions were answered and two null hypotheses were posited and tested at 0.05 level of significance. The mixed method research design was adopted for the study. The study was conducted in Rivers State with a sample size of 109 STEM teachers obtained using the purposive sampling method. Both qualitative and quantitative data were collected using “Structured Interview Template for Science, Technology, Engineering and Mathematics Teachers”, “Competency on Inclusive Education Questionnaire” [Cronbach's α = 0.86] and adopted instrument of “Multidimensional Attitudes Towards Inclusive Education Scale” (Mahat, 2008). Data was analyzed using Mean, Standard deviation, Analysis of Variance and Scheffe’s Post-hoc test. The findings of the study revealed that STEM teachers level of core competencies in the promotion of inclusive education in secondary schools is at the Average Level of Competence (ALC) [Aggregate Total Mean Value = 2.45, SD = 1.40] and their attitude towards the promotion of inclusive education in secondary schools is positive [Aggregate Total Mean Value = 3.30, SD = 1.38]. Empirically, it was proven that there is a significant difference between the mean values of STEM teachers’ core competencies and attitude respectively in the promotion of inclusive pedagogy in secondary schools [F(3,105) = 12.080, p = 0.000] and [F(3,105) = 15.058, p = 0.000]. Based on the findings, the study recommends that there is need for compulsory introduction of inclusive education core competence/attitudinal module in pre-service STEM teacher training programme in tertiary institutions. Refreshers course and trainings for in-service STEM teachers should be conducted on regular basis so that they can be acquainted with the contemporary approach towards inclusive education.