The move, according to WhatsApp, is intended to fight spam and abuse and improve the user experience.
"By coordinating more with Facebook, we'll be able to do things like track basic metrics about how often people use our services and better fight spam on WhatsApp," WhatsApp said in a blog post.
And by connecting phone numbers, Facebook can offer better friend suggestions and show more relevant ads, the post adds.
"Even as we coordinate more with Facebook in the months ahead, your encrypted messages stay private and no one else can read them. Not WhatsApp, not Facebook, nor anyone else," WhatsApp writes. "We won't post or share your WhatsApp number with others, including on Facebook, and we still won't sell, share, or give your phone number to advertisers."
WhatsApp subscribers without an active Facebook account are not required to sign up for the social network.
"Our belief in the value of private communications is unshakeable, and we remain committed to giving you the fastest, simplest, and most reliable experience on WhatsApp," the blog says.
This marks the first time in four years that WhatsApp has updated its terms and privacy policy.