The attack occurred around 8 p.m. outside the JR Nagano Station, the Kyodo and Jiji news agencies said, with police searching for the assailant.
The three victims were waiting at a bus stop, and were apparently targeted at random, Kyodo reported, citing an investigative source.
It said the man who died was aged 49, while another male victim in his 30s sustained serious injuries.
The third injured person was a woman in her 30s.
Nagano, the capital of Nagano Prefecture, lies some 225 km west of Tokyo and is a popular Japanese destination for winter sports.