Megabus will begin service from Philadelphia to Boston and eight other cities on July 21, with three departures a day from South Station.
Currently the only direct service the discounter offers from Boston is to New York, on double-decker buses.
To mark the expansion, Megabus, which offers fares as low as $1 and runs buses equipped with free Wi-Fi and power outlets, plans to offer 10,000 free seats for travel to and from Philadelphia from the nine cities.
Customers will need to use the promotional code LUVPHL when booking a ticket to have a chance at scoring a free seat.
Winners will be selected at random. There will not necessarily be free seats available on every bus each day.
Besides Boston, the new destination cities from Philadelphia are Atlantic City; Baltimore; Buffalo and Syracuse in New York; Toronto; and Harrisburg, Pittsburgh, and State College in Pennsylvania.