I own a hotel and had a look on booking.com for an API as I want to create my own front end interface to update my hotel room rates / numbers for some days. As well as upload pi
There is another 3rd party API that you may check, it is in following https://rapidapi.com/apidojo/api/booking, I have tested the API and their response data are as same as on the official site. The response are in JSON format. Currently, there are enough APIs end points that you need to query data, such as : Auto complete for locations, list properties with options and filters, get detail rooms, property description, facilities, policies, etc … . Hope this help.
Booking.com has an API but it is only available to "channel managers". I will not recommend a channel manager here but I will outline what you can do with it and what needs to be done:
Once you have setup your inventory (rateplan and roomtypes) on Booking.com you will need to do the following:
Finally, you will implement the channel manager's API to push information from your PMS to the channel. The channel will then push that information to Booking.com. You will have to login to the extranet every now and then to make sure all inventory and bookings are synced properly.
In Booking's admin page, have a look under > channel manager. I just signed up for a Planyo trial. Looks promising. They have a Booking extension as well.
There are at least two third-party channel management services that say they can synchronize via API with booking.com among many other aggregators:
1) http://verticalbooking.com/en/channel_manager.htm
2) http://beds24.com/channel-manager.html
So, presumably, there is a way to interface via API with booking.com but how to do that directly without a third-party I don't know...
UPDATE 3 - 9th Dec 2017
It would appear that Booking.com now have documentation available for their API, but you need to be signed up to them as a business in order to get access.
UPDATE 2 - 8th May 2017
It seems Booking.com have recently completed some Beta testing for their API, it may looks like they are making it even easier for people to connect. For more information regarding Booking.com's API, have a look here
UPDATE
Booking.com DO have an XML API system, however as many people have stated it is not available to Developers. It is internal and only made available to Hotel/Chains who are apart of their partner system. So YES they do, but NO it's not publicly available.
Original Answer
I can categorically state that at the time of writing, Booking.com do not have an API system, I know this because I've contacted them requesting such info, and they kindly advised me that they do not have an API that is currently available to developers.
I don't think booking.com offers API suitable for your business case. They use various techniques to get you to their booking system (like booking.com buttons) but as far as I know there is no way for common user to automate XML uploads or downloads, to sync your reservation system with booking.com, etc.