Reasoning ability is the 'problem solving skills' or 'analytical ability' or 'deductive and inductive reasoning'. Academic achievement is the total score one achieved at school, college, or university from class, laboratory, library, or field work. The objectives of the study were to explore the relationship between reasoning ability and academic achievement among secondary school students in Trivandrum district. Study also evaluated the difference in the mean scores of reasoning ability and academic achievement in terms of their background variables, such as gender, age, and type of school. Population of the study includes all students studying in secondary schools of Trivandrum district. The sample of the study consisted of 225 secondary school students of Trivandrum district. The tool adopted here was the Reasoning Ability of Secondary School Students Scale developed by the investigator herself and academic achievement by the term test conducted in the school by the state government. Statistical techniques involved here were t-test and Pearson's Product Moment coefficient of correlation. The result of the study showed that, there was significant high positive correlation between reasoning ability and academic achievement among secondary school students. Also male students showed more reasoning ability and academic achievement than that of female students. With reference to age, and type of school also there existed significant differences in reasoning ability and academic achievement. Educators need to recognize that the age and the type of the school are also influencing the student's reasoning ability and academic achievement of the students. Hence, it is necessary to allow the students to keep in touch with the latest facilities free of cost. So, the instructors have to arrange various competitive programmes, scientific exhibitions, various programs like quiz, talent test, puzzles, etc. Society allows students to access science and technology museums to students at low cost and encourages clubs and NGOs to arrange various programmes, such as seminars, symposium, etc., which promote the reasoning ability.