Any bodily movement produced by skeletal muscle which requires energy expenditure is known as physical activity (PA). WHO has recommended that at least 150 minutes of moderate or 75 minutes of vigorous-intensity PA or a combination of both per week is required for health benefits. Physical inactivity is one of the strongest risk factors for noncommunicable diseases (NCDs) and other conditions and is attributable to 6% of global premature death. However, data on the PA of teachers are unavailable in Nepal. They are considered one of the risk groups for NCDs because of the less active nature of their job. So, we aimed to evaluate the effect of the educational intervention based on the theory of planned behaviour on PA intention among secondary school teachers in Bhaktapur district, Nepal. For this study, we recruited 126 teachers from 6 schools. Each intervention and control group contained three randomly selected secondary schools. All the teachers from the selected schools were enrolled in the respective groups. A quasiexperimental (pretest-posttest control group) study design was used to test the effectiveness of the intervention on attitude, behaviour control, subjective norms, and intention for engaging in regular PA. Both groups underwent baseline and follow-up assessments at four weeks using the self-administered questionnaire developed for this study. The intervention group delivered a one-hour lecture session supported by audio-video materials for PA promotion. The effect was analysed by comparing the changes in the theory of planned behaviour (TPB) constructs within and between intervention and control groups. The difference in scores between and within the groups was tested using Student's t-test. Adjusted difference-in-difference scores were calculated through linear regression. Data analysis was done using Statistical Package for Social Science version 26.0. The adjusted mean score increase in TPB constructs due to the interaction of time and intervention increased from 0.641 to 1.381. The highest gain (beta = 1.381) was seen in the intention score, while a minor improvement was seen in perceived behavioural control (beta = 0.641). After the intervention, the net increase in PA intention score was 9.35% compared to the control group. Thus, the promotion package was effective in increasing PA intention. The findings of this study and educational package could be helpful in encouraging teachers to engage in PA in other schools.