Saṅghamittā (S. Saṃghamitrā; C. sengqiemiduo) (282 BC – 203 BCE) was a daughter of Emperor Ashoka and the sister of Mahinda. According to traditional accounts, Saṅghamittā was ordained as a nun (bhiksuni) at the age of eighteen years. Later, her father Ashoka sent her brother Mahinda to Sri Lanka, where Mahinda converted the king of Sri Lanka, Devānaṃpiyatissa, to Buddhism. Following this, Ashoka sent his daughter Saṅghamittā to Sri Lanka to ordain the first nuns there. Saṅghamittā traveled to Sri Lanka, bringing with her eleven other nuns and a branch of the the Bodhi tree.
Together the other nuns, Sanghamitta established a bhiksuni ordination lineage in Sri Lanka. This lineage eventually spread to the other countries of Southeast Asia (Thailand, Laos and Cambodia). The branch from the bodhi tree was planted in the Sri Lankan capital of Anurādhapura, and, according to tradition, remains to this day (known as the Jaya Sri Maha Bodhi).
Further reading: Sanghamitta (Wikipedia)