The explosion went off in Mount Lavinia town, 15 km away from Sri Lanka's largest city of Colombo.
A parcel bomb exploded when passengers started to leave the bus, with one of them spotting the boobytrapped parcel and alerting people.
All the injured passengers have been hospitalized.
Another bus hit a land mine two weeks ago. The authorities then put the blame on Tamil Tigers separatists, who have been fighting for independence of the country's northern and eastern areas, mostly populated by Tamils, for 25 years already.
The Sri Lankan government cancelled a cease fire agreement in January and launched an offensive against rebels in the country's north. Tamils have so far retained control of northern Sri Lanka and its center, Killinochchi.
