The 18-year-old winger joined Liverpool from Benfica in 2009, but has yet to make his debut and spent last season on loan at Northampton Town.
"He's a very talented player, but he's still raw," said Tykes manager Keith Hill. "We've watched him a few times and we've really been impressed with what we've seen.
"He went on loan to Northampton last season, which was a good experience for him. I spoke to Aidy Boothroyd about him and he agrees that he's an exciting prospect.
"Toni is left-footed and we've been looking for a left-footed winger for a while. But he can also play wide on the right and as a No.10, so he's the type of player we're looking for.
"This is a really good signing for the club but there needs to be some patience because he's still a young lad."