Marseille deputy prosecutor Christophe Barret told reporters the pensioner had turned up at the surgery with two handguns, a pistol and a revolver.
He ordered a patient being treated by the dentist out of the treatment room and immediately opened fire on the dentist, a woman in her 50s.
The shots resulted in the building being evacuated and police called. The presumed killer of the dentist was shot dead after he fired twice at officers, raising fears for the safety of at least two people who were trapped in an adjacent room.
A police source said the man had been in dispute with the practice over an unpaid bill of around 100 euros.
Although it appeared to be an isolated incident, Tuesday's shooting will add to Marseille's notoriety for gun crime and will further hamper the 2013 European City of Culture's efforts to forge a new image as a vibrant tourist destination.
The dentist's surgery was located in a tower block close to the Place Castellane, a popular central square full of restaurants and bars.
At least four young men have been killed this month in execution-style shootings linked to rival gangs battling for control of the drugs trade in the southern port's rundown estates.
A total of 24 people died in similar circumstances during 2012, giving Marseille one of the highest murder rates in Europe and prompting calls from some politicians for the army to be sent in.
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On Monday the Mayor of Marseille Jean-Claude Gaudin slammed the Socialist government in Paris for not investing enough money to help tackle Marseille's gun crime issue
On Monday the Mayor of Marseille Jean-Claude Gaudin slammed the Socialist government in Paris for not investing enough money to help tackle Marseille's gun crime issue.