The defender has previously played for several clubs in the I-League apart from representing the India U-23 team.
He turned out for Pune FC in the 2013-2014 season, his maiden season in the I-League -- and was on loan to reigning I-League champions Bengaluru FC last season.
"I have personally enjoyed Ranjan's game in the I-league matches and with his dedication and sincerity for football, find him to be the perfect fit in our club. I extend a warm welcome to him," NorthEast United FC owner John Abraham said in a statement.
NorthEast United's Director of Football Ardeshir Jeejeebhoy also welcomed the 20-year-old from Manipur to the ISL outfit.
"Salam has previously proved himself as a reliable center back. I am hoping with the right training, he would emerge as a valuable addition in the club," Jeejeebhoy said.
"I am glad to be signed by NEUFC and feel that it is the right place for me. Everyone at the club is immensely passionate about football and I look forward to training with the team and performing my best at the championship," Salam Ranjan Singh added.