The 20-year-old Norway international is under contract at Old Trafford until 2013 but has yet to establish himself in Sir Alex Ferguson's first team.
King spent time on loan last season at Hull City and has also in his career made temporary moves to Borussia Monchengladbach and Preston North End.
King's agent, Jim Solbakken: "I have discussed attacking options with Newcastle.
"There are many clubs who are interested in Joshua King but he is where he should be.
"He could have joined many clubs during the summer but got confidence in the Manchester United."