Chelsea have lost nine of 17 games in all competitions this season, leading to suggestions Mourinho could lose his job, and there have been rumours of dressing room unrest.
"I can assure you right now the players are 100% behind the manager," Terry said.
Mourinho was the subject of the first managerial vote of confidence in Roman Abramovich's 12-year ownership on October 5, but Chelsea have one win in five matches since ahead of Wednesday night's Champions League clash with Dynamo Kiev.
Asked how he and the players would feel if Mourinho did leave, Terry said: "It's not going to come to that
The club have shown faith in the best manager in the history of this football club.
He's by far, by a long, long way the very best.
"It's not what ifs
For me we're going to turn this around.
"I'm adamant we're going to turn this round and he'll be in charge for the rest of the season and long after I've finished playing at this football club."
Source : PA