Thānissaro Bhikkhu, also known as Ajaan Geoff (born 1949), is an American Theravada Buddhist monk of the Dhammayut Order (Dhammayutika Nikaya), Thai forest kammatthana tradition. He is currently the abbot of Metta Forest Monastery in San Diego County. Thānissaro Bhikkhu is a notably skilled and prolific translator of the Pāli Canon.He is also the author of many free Dhamma books.


Thānissaro Bhikkhu was born Geoffrey DeGraff in 1949 and was introduced to the Buddha's teaching on the Four Noble Truths as a high schooler, during a plane ride from the Philippines. After graduating in 1971 with a degree in European Intellectual History from Oberlin College, he travelled to Thailand, where he practiced meditation under Ajaan Fuang Jotiko, who'd studied under Ajaan Lee. He was ordained in 1976 at Ajaan Lee's Wat Asokaram, where Ajaan Lee's nephew, Phra Rajvarakhun (Samrong Gunavuddho), served as Preceptor for his ordination. Later, he took residence at Wat Dhammasathit in Thailand, where he continued studying under Ajaan Fuang.


Before Ajaan Fuang's death in 1986, he expressed his wish for Ajaan Geoff to become abbot of Wat Dhammasathit. Some time after his teacher's death, Ajaan Geoff was offered the position of abbot, but with "strings... attached" and no authority since he was a Westerner in a monastery founded by and for Thai monks. Instead of taking that position, he travelled to San Diego County in 1991, upon request of Ajaan Suwat Suvaco, where he helped start Metta Forest Monastery. He became abbot of the monastery in 1993.In 1995, Ajaan Geoff became the first American born, non-Thai bhikkhu to be given the title, authority, and responsibility of Preceptor (Upajjhaya) in the Dhammayut Order. He also serves as Treasurer of that order in the United States.