First Dorset drivers in Weymouth and Bridport voted by a margin of "about 70%" to turn down the deal, worth eight per cent over three years.
The Unite union said it was "very surprised" by the level of rejection.
It said it would put the firm on notice of further industrial action.
The drivers held a series of strikes between 20 June and 11 September, demanding pay equal to colleagues in other areas as well as improved terms and conditions.
Unite said the latest offer would have raised the hourly rate from Â£8.80 to Â£9.50, the same level as First Dorset drivers in parts of Somerset, although well below salaries paid by some other firms in Dorset.
Regional officer Bob Lanning said: "The offer met 98% of our members' expectations. I thought we'd cracked it but clearly I have more work to do."
He said members had already approved further strikes, which were likely to start in two weeks' time.
First Dorset ran emergency timetables during the summer strikes, which it said involved about 60% of its drivers.
Managing director, Marc Reddy, said: "It's disappointing that despite meeting the vast majority of the trade unions' aspirations during the negotiations that the Unite members rejected the revised offer.  We remain open to negotiations at any time."
Bus drivers who held a series of strikes over the summer are "very likely" to renew their action after rejecting a "substantially improved" pay offer, their union has said.