Vajji (Sanskrit: Vṛji) or Vrijji was one of the principal Mahajanapadas of ancient India. Both the Buddhist text Anguttara Nikaya and the Jaina text Bhagavati Sutra (Saya xv Uddesa I) included Vajji in their lists of solasa (sixteen) Maha-Janapadas. The name of this Mahajanapada was derived from one of its ruling clans, the Vṛjis. The Vajji state is inidicated to have been a republic. This clan is mentioned by Pāṇini, Kautilya and Xuanzang.

The territory of the Vajji Maha-Janapada was located on the north of the Ganges River and extended up to the Terai region of Nepal. On the west, the Gandak River was probably the boundary between it and the Malla Maha-Janapada and possibly also separated it with the Kosala Maha-Janapada. On the east, its territory was probably extended up to the forests along the banks of the rivers, Koshi and Mahananda. The capital of this Maha-Janapada was Vaishali. Other important towns and villages were Kundapura or Kundagrama (a suburb of Vaishali), Bhoganagara and Hatthigama.