Steve Bruce’s side fell behind on five minutes when Mohamed Elneny scored his first-ever top-flight goal.
Pierre-Emerick Aubameyang then continued his goalscoring run against the Magpies as the Gunners cruised to victory.
United remain nine points above the relegation zone ahead of the final four fixtures.
Our reporter Miles Starforth was at St James’s Park – and here’s how he rated each Newcastle player on a disappointing afternoon:
5. Paul Dummett - 5
Steady performance on an awkward afternoon for United's defence Photo: JOHN WALTON
6. Matt Ritchie - 6
United's best player, was keen to get the team on to the front foot, tried to inject some energy into the team, and hassled and harried Arsenal Photo: Stu Forster
7. Jonjo Shelvey - 4
Hit and miss with his first-half passes in an over-run United midfield. Photo: Pool
8. Sean Longstaff - 5
Handed a five successive start, nowhere near his best, though was neater and tidier in possession than some of his team-mates. Photo: Stu Forster