It was first reported by Collider.com that the second installment in Marvel's Guardians of the Galaxy series would be titled Guardians of the Galaxy Vol. 2, and it turned out it's the truth: it was confirmed by its director James Gunn on twitter that this is indeed how the upcoming movie will be called.
Collider broke the news when Kevin Feige (Marvel Studio's head honcho) accidentally dropped the title in an interview.
When Collider reported the slip-up, James Gunn replied with the following tweet:
Yes, the official title for the @Guardians sequel is GUARDIANS OF THE GALAXY VOL. 2. Nice detective work, @colliderfrosty, etc! You got us!— James Gunn (@JamesGunn) June 29, 2015
It's interesting that Marvel associates a number with the Guardians of the Galaxy sequel: outside of the Iron Man franchise, all of their films have had a subtitle: rather than being called Captain America 2, the second Steve Rogers adventures were called Captain America: The Winter Soldier. Thor 2 wasn't Thor 2, but Thor: The Dark World. And so on…
But this makes sense, as this title is a nice nod to the mixtapes Star Lord's mother gave to his son (Awesome Mix Vol. 1 & 2).
Guardians of the Galaxy Vol. 2 will be out on the 28th April (UK) / 5th May (US), 2017.