Gusii or Kisii is one of the Bantu languages spoken by perhaps 2,000,000 people in the Kisii highlands (two thousand meter elevation) in the Nyanza province of Kenya.

Gusii is remarkable for the low number of words that it shares with other members of the Bantu family, even with its closely related neighbors such as Luyia. Derek Nurse described Gusii as having "one of the most interfered-with vocabularies in East Africa". It seems to have borrowed from Cushitic languages at some earlier period, and more recently from Nilo-Saharan languages. Its territory is entirely surrounded by the Luo languages, Kipsigis of the Kalenjin languages, and Masai.