At a Q&A session at MetroCon, voice actor Nolan North confirmed the existence of a second The Last of Us game. He also stated that his character won't be in it, which, without spoiling things, makes sense given that character's arc, and that it seems unlikely that The Last of Us 2 would be a direct sequel to the first game. The plot of The Last of Us wrapped up nicely, and it would take one hell of a new story (or some kind of prequel) to have bringing Joel and Ellie back make sense.
Meanwhile at Indy PopCon, Joel's voice actor, Troy Baker, claims ignorance on the topic. Someone asked what Baker thought a sequel would be about, and he prefaced his answer by saying "I know nothing. I don't know if we're gonna do another one or not" but that he trusts Naughty Dog to tell a worthwhile story. If you're wondering, Baker would like to see Joel's story during the 20 years between the prologue and the main game.
Troy Baker's comments certainly don't mean Nolan North is wrong; Baker might just be better at keeping a secret, or way less informed than North. Are you excited for another The Last of Us game?