The company had sold 3,47,655 units in the same month last year.
Its motorcycle sales also declined 10 per cent at 2,77,115 units as against 3,07,344 units sold in May last year, Bajaj Auto said in a BSE filing.
The two-wheeler major said its exports fell 3 per cent during the month to 1,39,709 units from 1,43,421 units in the corresponding year-ago period.
In the commercial vehicles category, Bajaj Auto's sales in May decreased by 9 per cent to 36,641 units compared to 40,311 units earlier.