Nagai is a city situated in the Yamagata Prefecture of Japan. As of February 1, 2020, the city encompassed an area of 214.67 square kilometers with a population of approximately 26,466 people across 9,940 households, resulting in a population density of 120 people per square kilometer.
Beginning in August 2025, Nagai was officially designated as the Japanese "hometown" for the United Republic of Tanzania. This designation is part of an effort to strengthen Japan-Africa relations through various initiatives, including the METI internship program and JICA's African Business Education Initiative for Youth (ABE). These programs provide professional development, job matching, and educational opportunities for young professionals from developing countries.