Skipper Terry has made no secret of his wish to extend his stay into an 18th season in Stamford Bridge first-team fold.
The 34-year-old has been in superb form for Chelsea this campaign and was on the scoresheet in Sunday's 2-0 victory over Tottenham in the League Cup final.
Manager Mourinho has spoken previously of his desire for Terry to remain at the club and believes a new deal is a formality.
"Before the end of the season for sure, what I can guarantee is that he will be a Chelsea player in the next season I can guarantee that," Mourinho said.
"I can guarantee that because I know what my board tells me, I know what the player tells me. [There is] no doubt he's going to get his contract."
Mourinho admitted it would be hard to find someone who could fill Terry's boots, but he feels for now the defender has what it takes to continue.
"Every one of these big guys are difficult to replace," he added. "I think at the same time you have the player, that player is still an important player, you have to start thinking what next?
"I think that's the best way to do things, sometimes you have the solution just around the corner, sometimes you have to look outside your club.
'"It's a process we can take our time with because John is proving again this season [how important he is].
"When I came back last year he was coming from a difficult period, but last season he was very good.
"This season even better than last season. He has time to enjoy his football with us and he has given us time to think about a replacement."
