Rakuten, one of Japan’s largest Internet companies, has announced that it will acquire Cyprus-based messaging platform and app maker Viber for $900 million.

The deal is the latest and largest in an aggressive acquisition spree by Rakuten as it seeks to “become the world’s No. 1 Internet services company,” it said, and marks its entry into the global messaging market.

Viber can now potentially add Rakuten Group’s approximately 225 million members to its existing user base of 300 million registered users.

The Rakuten-Viber pairing may pose a significant new challenge to Line, which announced that had 300 million registered users back in November 2013 and is aiming for the 500 million milestone in 2014. Like Line, Viber users can also purchase and send stickers to one another, and it recently added Viber Out, which enables free calls to mobile and landline numbers. Viber’s apps do not currently have games, one of Line’s key draws, but CEO Hiroshi Mikitani hinted that games are part of Rakuten’s plans for the messaging service.

In a statement, Mikitani said:

I am tremendously excited to welcome Viber to the Rakuten family. Viber delivers the most consistently high quality and convenient messaging and VoIP experience available. Additionally, Viber has introduced a great sticker market and has tremendous potential as a gaming platform.

Over the last two years, Rakuten’s acquisitions have included: e-reading platform Kobo for $315 million in cash; Spanish streaming video service Wuaki.tv for an undisclosed amount; and Viki, a global video streaming platform that crowdsources translated subtitles, for a reported $200 million.

In addition to its shopping spree, Rakuten was also a lead investor in the $100 million round that valued Pinterest at $1.5 billion in May 2012. It has also set up a $10 million fund to invest in startups primarily from Southeast Asia, which has one of the world’s fastest-growing mobile marketplaces, as well as Taiwan.

The news comes just two days after Viber Media denied a report that it was in talks to be acquired by an instant messaging company from Asia for $300 million to $400 million.

Update: Commenting on the acquisition to TechCrunch via email, Viber CEO and founder Talmon Marco declined to answer specific questions about the future of his role and Viber’s service under Rakuten, saying only: “We will continue focusing on building the best messaging and voice service out there. Lots of new things in the funnel for 2014. It’s going to be the most exciting year for Viber thus far!”

TechCrunch’s Natasha Lomas contributed to this report