Over the past three decades, several attempts have been made to juxtapose or connect Bionian theories with Buddhism. Previous studies have merely speculated on the associations between Bion and Buddhism - whether remarking on their similarities or drawing distinctions between them. In contrast, this article seeks to provide concrete evidence of Bion's deep interest in Zen Buddhism by analysing the annotations and underlinings in his personal copy of Alan Watts's The Way of Zen in order to shed further light on those aspects of Zen which most engaged Bion's attention. Five main themes are explored in detail - the ‘Four Noble Truths’; conventional knowledge versus spontaneity; non-verbal communication; ‘no memory no desire’; and ‘O’ (or ‘ultimate reality’) - in order to provide a clearer picture of the elements which attracted Bion's interest in Buddhism and the places where Buddhism resonated with his own psychoanalytic work.