Who was the first monk ordained by the Buddha?

The first monk ordained by the Buddha was Kondanna, also known as Kaundinya or Añña Koṇḍañña.

Kondanna was one of the five ascetics who had practiced severe austerities with Siddhartha Gautama before the Buddha’s enlightenment. After enlightenment, the Buddha travelled to Sarnath and delivered his first sermon, traditionally called the Dhammacakkappavattana Sutta. Kondanna understood the teaching first, prompting the Buddha’s celebrated recognition that he had understood.

A few days later, after hearing the Anattalakkhana Sutta, Kondanna attained arahantship. He then requested admission to the Buddha’s new monastic community. The Buddha ordained him with the words “Come, monk,” making him the first bhikkhu in the Buddha’s dispensation and the first member of the Sangha.

A common mix-up is between being the first monk ordained and being the first person to understand the teaching: in Kondanna’s case, traditional accounts identify him as both the first to comprehend the Dhamma and the first monk ordained. Sariputta and Moggallana became chief disciples later, while Ananda joined the Sangha afterward.