Although prostitution is legal in France, soliciting in public and pimping are not.
The National Assembly voted 16 months ago to impose a â‚¬1,500 (Â£1,090; $1,600) fine on clients, shifting the criminal burden from prostitutes.
The upper house's decision to scrap the fine will now go back to the assembly.
Social affairs minister Marisol Touraine said that Monday night's vote was "absolutely unbelievable and contemptuous towards women".
But Joelle Garriaud-Maylam, a conservative senator, argued that the offence of soliciting was a useful resource for the authorities. "To help these women, you first of all have to identify them."
The initial move to shift penalties from prostitutes to their clients followed a Swedish law that made it a crime to buy sex but not to sell it. Supporters argued that the move would help tackle trafficking networks.
A Swedish government-commissioned report suggested that the 1999 law had brought about a dramatic fall in the numbers of women working as prostitutes. But the effect of the law is not entirely clear as many prostitutes have moved off the streets and on to the internet.
France has an estimated 30,000 sex workers and the interior ministry says most come from eastern Europe, Africa, China and South America.
Socialists who backed France's 2013 bill to criminalise paying for sex, rather than soliciting for it, had initially hoped that the trial of former International Monetary Fund chief Dominique Strauss-Kahn might influence the conservative-dominated Senate.
Maud Olivier, a Socialist MP, said the trial had shone a light on the truth behind the world of escorts. "There is no choice; violence is ever present," she told Le Monde newspaper.
The trial was bruising for Mr Strauss-Kahn, who endured a torrid time facing accusations of aggravated pimping, as several women gave evidence against him before dropping their case.
He denied knowing the women were prostitutes. The verdict is expected in June.
France's Senate has rejected a planned 2013 law penalising people who pay for sex, and has chosen to maintain the offence of soliciting.