The patron of Castelletto Merli is Sant'Eusebio di Vercelli, celebrated on 1 August; the village holds the Chiesa di Sant'Eusebio.
In the second weekend of June the Festa dei Merli is celebrated, a food and wine festival with an inter-provincial meeting of the Piedmontese cattle breed, organised by the Pro Loco at the municipal sports ground. Since 2012 the Festival di Teatro Amatoriale «Castelletto Merli Palcoscenico» (amateur theatre festival), organised by the Associazione Io Vivo Castelletto, has been held, an event lasting three days. The municipality also holds the Chiesa di Sant'Antonio and the Chiesa del Santissimo Nome di Maria.
