Several sources have confirmed the discussions as the French club prepare for a Champions League campaign next season by making an approach for the 22-year-old.
Tomori had initially suffered from injury upon the Premier League restarting in June after the coronavirus-enforced shutdown but he has been available in the last few weeks without being used by manager Frank Lampard.
The situation has led to concern but he has yet to discuss Lampard's long-term plans for him and whether Chelsea believe a season away from the club can help his long-term prospects in west London.
The Blues have signed up a host of academy players to new contracts this season, including Tomori, who agreed on a new five-year deal in December after his breakthrough season under Lampard.
However, Chelsea are open to changes in defence having conceded 54 goals in the league this season, which is the worst figure since Roman Abramovich took over the club in 2003.
Alongside Tomori's interest from Rennes, Kurt Zouma's entourage has reportedly had contact with Paris Saint-Germain over a possible summer move.
All four defenders would happily remain at Chelsea but Lampard opened the door to changes after the 2-1 loss to Arsenal in the FA Cup final last Saturday.
“Yeah, it’s true," Lampard said. "The numbers don’t lie. We are trying to play a certain way. I know that teams at the top of the game, Liverpool and Manchester City, have been through similar. I’m not saying we are going to get to that level, but there were similar periods in the early stages of their managers being at the club a few years ago.
"We are conceding too much; it is a work in progress for me. I can’t shelve it in another direction. We have to work defensively as a group. But some of the individual errors we are making, clear individual errors, some today clearly cost us goals, can’t be there if we want to keep moving forward.
"We have to work as a group to make that better. That is clear; we are not hiding from that. We have come fourth, we’ve had a good season. There are areas to improve and defensively we have to improve.”
Alternatives include Atletico Madrid's Jan Oblak, although Diego Simeone would want the full €120m release clause (£110m/$137.5m) paid. There's also interest in Ajax's Andre Onana who is available for a significantly lower fee and who would welcome a move to London in the upcoming transfer window.
Chelsea next face Bayern Munich in the Champions League at the Allianz Arena on Saturday, having lost the first leg at home 3-0. They face a selection crisis with up to 10 players out and have called upon a host of youngsters including Ian Maatsen, Lewis Bate and Dynel Simeu.