Niinivesi is a medium-sized lake of Finland within the Kymijoki main catchment area. The lake is situated in the municipalities of Rautalampi, Tervo and Vesanto. Niinivesi is almost divided into a northern and southern part. The northern part is 20 kilometers long, resembling a narrow bay. The southern part is connected westwards to the lake Kiesimä via the Kerkonkoski Canal. The water quality in the lake is excellent and clear.