It's not official but it's a darn good indication.
During the 2015 Taipei Game Show, Sony showed off a sizzle reel of PlayStation 4 games. That's when attendees saw a very interesting image: The number "2" in the style of the Metal Gear Rising: Revengeance logo.
It appears for only a brief second but it's definitely there, which means we should probably anticipate a sequel to the well-received spin-off. You can see it for yourself during the Twitch stream ; it occurs a little after the 2:46 mark. As for MGS creator Hideo Kojima, he didn't say anything about the project, although rumor has it he's been meeting with Revengeance developer Platinum Games recently.
Metal Gear Rising: Revengeance was a really fun game. Can't wait to hear about a follow-up effort!
Related Game(s): Metal Gear Rising: Revengeance