Igor Thiago, Ethan Pinnock, Dango Ouattara and Callum Wilson were all on target as Keith Andrews' side won an entertaining six-goal contest at Roazhon Park, with the Premier League curtain-raiser against Tottenham Hotspur now just two weeks away.
Thiago made his first appearance of pre-season after returning late to the group following Brazil's World Cup campaign, heading Brentford in front from a Mathias Jensen corner on 19 minutes before Ethan Pinnock doubled the lead nine minutes later.
Rennes pulled a goal back through Sebastian Szymański within a minute of the restart, but Wilson, on for Thiago at half-time, quickly restored the two-goal cushion, setting up Ouattara before scoring himself two minutes later. Valentin Rongier's close-range volley completed the scoring for the hosts on 71 minutes.
Wilson reflected positively on the performance afterwards.
"We're delighted. It was a tough test in this weather against a good team," he said. "As the game went on, you could see people getting tired and mistakes happening, which let them back in, but overall it was a good performance and a good win.
"I was delighted to get on the scoresheet and to help other people, but I'm disappointed I didn't score more. It's a feeling of frustration, but that's the striker in me."
Brentford complete their pre-season schedule against Eintracht Frankfurt next Saturday, with the German side visiting Gtech Community Stadium.
Away from the first team, Brentford confirmed forward Iwan Morgan has joined Grimsby Town on a permanent deal after three years at the club, signing a three-year contract until 2029 for an undisclosed fee.
The 20-year-old Wales youth international scored 25 goals in 42 appearances for Brentford B last season as the young Bees won the Professional Development League title, before spending the second half of the campaign on loan at Shrewsbury Town, where he made 22 appearances and scored three goals.
Grimsby manager Dave Artell said the club had long admired the forward.
"He's a player we've admired for a while and somebody we believe has enormous potential," Artell told the club website.