The Old Trinitarians Union (OTU) is an incorporated association of Trinity Grammar School Old Boys, governed by Old Boys. The Objects of the OTU are:

1. to further the interests and ideals of the School;
2. to further the connection between Old Boys of the School and the interest of Old Boys in the School;
3. to further the spiritual, moral, social and intellectual welfare of its members; and
4. to support such charitable institutions and objects from time to time and in such a manner as it may in its absolute discretion think fit.
   
   

The OTU is supported by the Alumni Office of the School.