The man was riding a white Yamaha motorbike along Barnsley Road in South Elmsall when the incident happened at 6.50pm on Wednesday.
Police said the bike was heading in the direction of South Elmsall from South Kirkby and the single decker bus was on the other side of the road.
The rider suffered serious injuries and was taken to hospital by air ambulance.
Anyone with any information is asked to contact the Major Collision Enquiry Team via 101 quoting log number *1590 of 18 April.
Details can also be passed via the West Yorkshire Police website at www.westyorkshire.police.uk/contact-us