The Premier League club have the option to make the deal permanent if the Argentine impresses at St James' Park.
Ferreyra joined Shakhtar in July last year after scoring 17 goals in 25 appearances for Velez Sarsfield in his homeland.
The 23-year-old continued his strong form in the Ukrainian top flight, finding the net six times in 13 league games as Shakhtar secured a fifth straight title.
However, Ferreyra caused controversy last month as he and five other players refused to return to Shakhtar for pre-season training due to the civil and political unrest in Ukraine.
He later apologised for his actions, but will be playing his football away from Ukraine after becoming Newcastle's seventh acquisition of the transfer window.
The former Argentina Under-20 international has joined up with the Newcastle squad in Germany to take part in the Schalke Cup and revealed he had no hesitation in making the move.
"I am very pleased to sign for Newcastle United," Ferreyra told Newcastle's official website.
"I like the Premier League and when I knew that Newcastle were interested in me, it was a very easy decision to make.
"I spoke to my friend [Newcastle captain] Fabricio Coloccini, and when he told me about the club, I didn't need to think twice."
Newcastle manager Alan Pardew added: "Facundo is someone we know on a number of levels, including his friendship with Fabricio, and we think he could be a big success here at Newcastle.
"This is a player who has played at the top level and is highly-regarded in Argentina.
"He has tremendous potential and hopefully he can realise that here at Newcastle United."
Pardew has already added the likes of Emmanuel Riviere, Remy Cabella, Siem de Jong and Daryl Janmaat to his squad as he looks to build on a 10th-placed finish last season.
