Established in 1970, the Student Union of St. Catharine's has done a lot to promote the welfare of the students.

The Student Union aims at cultivating St. Catharine's spirit among the students: be brave, gentle and sincere. Besides, it acts as a communication channel between the students and the school. It also organizes a wide variety of extra-curricular activities for both amusement and academic purposes.

The executive committee of the Student Union comprises thirteen committee members. Nine of them are elected by all students. Four members, including two treasurers and the representatives of the Fellowship and the Discipline group, are appointed by teachers. There are ten teacher advisors working with the executive committee.