Kinnear was admitted "as a precautionary measure" and would miss this afternoon's League game at West Brom.
Assistant boss Chris Hughton, who acted as caretaker manager before Kinnear was appointed, will take charge of the team at The Hawthorns.
NUFC: "This morning, Joe Kinnear reported feeling unwell at the team hotel ahead of Newcastle United's game at West Bromwich Albion.
"After being examined by the club's medical staff, Joe was taken to hospital, as a precautionary measure, where he remains under observation.
"Joe will miss the game, with Chris Hughton taking charge of the team at The Hawthorns."