Neil Lennon's men had already reached the knockout stage after matchday four, but Lazio's victory against Cluj ensured the Hoops will finish at the summit of a European group table for the first time.
Already-eliminated Rennes scarcely looked capable of stalling the hosts and Lewis Morgan and Ryan Christie had the Scottish Premiership champion two up at the break.
Mikey Johnston came off the bench to add a third, while fellow substitute Adrien Hunou grabbed a consolation for Rennes, but Celtic can look forward to a trip to Cluj with little on the line, while domestic title rivals Rangers remain engaged in a tense group-stage tussle.
Faced with fairly lacklustre opponents, Celtic soon took control and a pair of smart Edouard Mendy saves from Olivier Ntcham efforts sandwiched the opening goal.
James Forrest cut the ball back from the left-hand side of the penalty area for Morgan to provide a devastating low finish at the near post after 21 minutes.
A deserved second followed in first-half stoppage time as Mendy failed to get a firm hand to Christie's 20-yard drive and saw the ball skip into the net.
Perhaps unsurprisingly, Celtic lacked urgency following the restart and were grateful to Forster for a magnificent triple-save.
The former England goalkeeper instinctively blocked from Flavien Tait inside the six-yard box, leapt on Jordan Siebatcheu's rebound to deflect the ball onto the post, then sprung to his feet to gather a tame James Lea Siliki attempt.
Johnston ensured the points were safe, setting off on a dazzling run from left to right and finishing into the far corner, before Rennes finally got their goal with an 89th-minute Hunou header.