That means users with the app downloaded on their phone can now pair their device with the web version to write messages via their browser.
The web version is an extension of WhatsApp for mobile, rather than a standalone browser service, and there are limitations.
Due to "Apple platform limitations", iPhone users will not be able to use the web service. WhatsApp Web is available to Android, BlackBerry, Windows Phone and certain Symbian devices.
Users will need the latest version of WhatsApp running on their phones, and they also need to be using the Google Chrome browser. They can then visit web.whatsapp.com to set up the web version. Users will need to scan the QR code displayed on the page to pair their devices.
WhatsApp, which is owned by Facebook, is the biggest messaging app globally at 700m users sending around 30bn messages a day. Analysts believe that while there are more SMS users, there are more WhatsApp messages being sent than traditional texts.
WhatsApp message volume growth is still accelerating. Has probably now overtaken SMS. pic.twitter.com/KsR85Mplrt— Benedict Evans (@BenedictEvans) January 20, 2014
Given Facebook's ongoing mission to bring the world online, it's likely that bringing WhatsApp to the web is likely to boost engagement and message volume even further.