The governor of Tungurahua province, Lira Villalba, told reporters that the bus went off the road between the provincial capital Ambato and the town of Pillaro.
Initial investigations have shown brake failure was the likely cause.
Road accidents are a serious problem in Ecuador, with an annual average of 33 deaths per 100,000 residents – one of the highest mortality rates in Latin America.
Over the last four years, an average of about 4,800 people have died each year in road accidents in the Andean nation – or 13 people a day, according to road safety watchdog group Justicia Vial.