William of Saint-Thierry presents a Trinitarian theology of deification in his Exposition on the Song of Songs. He spiritualizes the erotic imageries of the Song of Songs to present his Trinitarian theology of deification. Comparing his theology of deification in the Exposition to his previous works on deification, we are able to see that there is a shift from a Pneumatological focus in On Contemplating God and Meditations to a Trinitarian framework in the Exposition. This, I argue, is partly due to his spiritual reading of the Song of Songs which contributes to his forming of his Trinitarian theology of deification. And whilst William is original in his reading a Trinitarian theology of deification from the Song of Songs, I will also explain how he is heavily influenced by Augustine's psychological analogy of the Trinity. In this paper, I will present how the erotic imageries of the Song of Songs contributed to William's understanding of the Trinitarian theology of deification, the Trinitarian image of God in the human person, the correspondence of the faculties of the mind to the appropriations of the Trinity, the three theological virtues, and the biblical historical events of the Passion, Resurrection and Pentecost. I will delve into William's texts to show how he interprets certain imageries and narratives to craft a Trinitarian theology of deification - which includes his understanding of anthropology, theological virtues, unity with God etc. At the end, I will also put St Bernard of Clairvaux's Sermons on the Song of Songs in dialogue with William of Saint-Thierry, comparing how they discuss the Trinity in their respective interpretations of the Song of Songs.