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.

Police, fire and ambulance personnel were called to scene at about 15:00. They were assisted by a coastguard rescue helicopter.
Unconfirmed reports have said a Ford Transit van and a campervan collided head-on.
One person is thought to have been taken to hospital by helicopter with serious injuries.
Traffic Scotland has warned drivers in the area to expect delays.

The Royal Borough of Windsor and Maidenhead is one of the first councils in England to launch a PledgeBank.
The pledges, from cleaning graffiti to organising a street party, can only be undertaken if other residents commit to the idea.
The Conservative council said the scheme helps it save money but also "results in a better social outcome".
Once a pledge is made, the council will check it fits the set criteria and then publish it online.
It comes after a failed Big Society scheme to get residents to donate savings in their council tax, which the council has cut for the past four years.
George Bathurst, cabinet member for policy and performance, said other schemes such as Green Redeem recycling rewards were a success.
He said: "No one wants to just hand over a cheque but everybody would like to do something for their community."
Mr Bathurst will make the first pledge to donate five Christmas food items to FoodBank if 10 other local people will do the same by 18 December.
Fiona Dent, Windsor's Labour candidate, said: "Volunteering creates strong bonds between people, reinforces their ideals and can provide the help people need in difficult times.
"However, we are living through difficult times now. Most people in the middle and lower incomes are a lot worse off than they were five years ago and they may well have more than one job to help pay the bills, leaving less time for volunteer activities."
France's Senate has rejected a planned 2013 law penalising people who pay for sex, and has chosen to maintain the offence of soliciting.
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The A9 Perth to Inverness road has been closed in both directions following an accident near Kincraig.
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People in part of Berkshire are being encouraged to make pledges to help their community.