The left-sided 17-cap Northern Ireland international, who can play in either defence or midfield, will remain at The Den until Saturday, 7th November.
Twenty-four-year-old Ferguson was born in Derry, in Northern Ireland, but came up through the ranks on Tyneside before making his Magpies’ debut in August 2010, at Accrington Stanley in the League Cup.
He has made 32 appearances in all competitions for Newcastle, with 19 of those coming as a substitute.
Ferguson has also spent time on loan at Birmingham City and Glasgow Rangers over the last three seasons.
Millwall, who were relegated from the Championship last term, begin their 2015/16 League One campaign away at Shrewsbury Town on Saturday.
7 August 2015
2015-16 season tickets are on sale now. Adult tickets available from £525 and juniors from £250. Call 0844 372 1892 or visitwww.nufc.co.uk