The 23-year-old linked up with the Championship side on the final day of the January transfer window, and will stay at the Hawthorns for the remainder of the current campaign.
He joins fellow Magpie Dwight Gayle – who is also on loan at the Baggies from Newcastle – in Darren Moore’s squad.
Murphy signed for United from Norwich City in the summer of 2017 and went on to make 24 appearances in his first season at the club, scoring once against Manchester City.
So far this term, he has played 13 times for Rafa Benítez’s side, most recently in the 2-0 FA Cup defeat to Watford last weekend.
London-born Murphy could make his West Brom debut in their home game against Middlesbrough on Saturday afternoon.