Fresno firefighters responded to the incident on the bus in the area of Tuolumne and P streets at about 1 p.m. on Wednesday.
The 53-year-old man told the crew he had an electronic battery-powered smoking device in his right pant pocket that suddenly became hot before sparks went flying and the e-cigarette exploded.
Other people on the bus said the man began jumping around.
Paramedics took the man to the hospital with burns to his right thigh and hand. He was expected to be OK.
No one else on the bus was injured.
Download the free ABC7 Los Angeles app for breaking news, weather and local stories on-the-go