There is only one direct bus operating on this route. This is the sleeper bus of Virak Buntham, which will take you in about 16 hours to Kep. Another option is to take a morning bus to Phnom Penh and take another bus from there to Kep.
Taking the taxi is a lot faster than the night bus, but comes at a higher price with fares starting at $125.
There are also a few flights leaving around noon from Siem Reap to Sihanoukville. Upon arrival, you can take a taxi from the airport, which will bring you in 3 hours to Kep.
The bus departs from the terminal in the south of Siem Reap next to the Nakpeoun Old Market roundabout. It arrives at Virak Buntham Express, Kep Stop.
Siem Reap to Kep by Indirect bus
Costs: $10 + $8, duration: 6 + 4 hours
If you don’t want to travel at night, you can also take a day bus to Phnom Penh first and then catch another bus to Kep.
Step 1: Siem Reap to Phnom Penh by Bus
The bus journey from Siem Reap to Phnom Penh takes about 6 hours and costs $10.50-$14. Companies operating this route like Cambodia Post VIP Van and Mekong Express have a better reputation than Virak Buntham.
See the page of Siem Reap to Phnom Penh for more info about this route. You can also book tickets directly in the link below.