Japan Railways (JR) operates a bus from Niigata to Sendai hourly. Tickets cost $1 - $2 and the journey takes 10 min. JR Bus Tōhoku also services this route 4 times a day.
How to get from Niigata to Sendai
From Aoyama(Niigata) train to Shin-Niigata then bus to Hirosedori. 5 h 15 min ¥4714. From Aoyama(Niigata) train to Niigata Mandai Guchi BT, bus to Yamagata Yamako Bldg, then bus to Hirose dori Ichiban-cho. 6 h 12 min ¥5048. From Niigata drive to Sendai. 3 h 12 min ¥3108.