This study validated an instrument for measuring student engagement in the context of Chinese science curriculum reforms. We adapted existing items from well-designed instruments with the context of science learning and formed the initial instrument of the Student Engagement in Science Learning (SESL) consisting of 20 items in four subscales, including cognitive, emotional, behavioural, and agentic engagement. We recruited an expert panel to provide content and face validity during the adaptation process. The final sample of 578 was collected and analysed to provide the construct validity of the SESL instrument. The results from item analysis, confirmatory factor analysis, correlation analysis conformed the instrument's convergent and discriminant validity. Independent t-tests of gender and district groups confirmed the concurrent validity of the instrument. The Cronbach's coefficient alpha of the four subscales and the total scale are higher than 0.70, indicating the SESL instrument has the internal consistency reliability for measuring student engagement in science learning. Researchers and practitioners would benefit from the valid and reliable SESL instrument with the four constructs to measure and improve student engagement in science learning.