The 23-year-old, who was out of contract at the end of last month, put pen to paper on a two-year deal before heading for South Wales.
A club statement said: "Sammy Ameobi has joined Cardiff City on a season-long loan deal, after signing a new contract with Newcastle United.
"The 23-year-old winger has put pen-to-paper on a two-year deal at St. James' Park, and will now spend the 2015/16 campaign in South Wales, turning out for the Bluebirds in the Championship."