Merlier, who won stage seven with a similar late surge on Friday, waited till the last 400 metres to make his move. His raw power was enough to carry him past Jasper Philipsen, Biniam Girmay and others who had positioned themselves for the sprint finish.
Four-time champion Tadej Pogacar (UAE Team Emirates-XRG) retained his overall lead after the flat 180.4-km ride from Perigueux to Bergerac, while Jonas Vingegaard (Visma-Lease a Bike) stayed second in the race for the yellow jersey, 2 minutes and 42 seconds off the lead.
"Just before the last corner I was a bit boxed in, then I almost crashed - I thought it was over... With 250m to go I say, 'I'll give it a try and we will see'," said 33-year-old Merlier.
Eritrean Girmay (NSN) finished second in the stage, while Merlier's compatriot Philipsen (Alpecin-Premier Tech) came in fourth behind stage five winner Olav Kooij.
Thibault Guernalec, Jakub Otruba and Liam Slock made an early breakaway while the general classification contenders stayed back in the peloton.
Belgian Slock (Lotto-Intermarche), who made his Grand Tour debut at the Vuelta a Espana last year, launched an attack during the climb to Buisson de Cadouin and took a solo lead with about 40km left.
The peloton began to chip away at his lead in the last 10 kilometres, with the NSN, Decathlon-CMA CGM and Uno-X Mobility riders taking turns at the front of the group, and Slock was finally caught just before the final kilometre.
After a final right turn, the Alpecin–Premier Tech team set up Philipsen for the final sprint. But Merlier was waiting, about 10 riders behind, for the moment to launch his attack.
"In the last 50m, I couldn’t push anymore," said Merlier. "In this heat it was a really difficult effort."