The nine senators from the UDI-UC party wrote a joint op-ed in the Journal du Dimanche weekly to say they would support Macron, a former minister in Socialist President Francois Hollande's government, because of his pro-European stance and bid to go beyond the Left-Right political divide.
"Emmanuel Macron's method is the right one," they wrote, according to Reuters.
"He wants to bring people together ... and trigger a new dialogue between the French people and their representatives."
Fillon was the frontrunner for France's April and May presidential election until an investigative weekly reported in late January that he had paid his wife as his parliamentary assistant for work she did not do. He denies any wrongdoing but magistrates put him under investigation, a first for a presidential candidate in France.
Macron, an independent centrist who created his own En Marche! (Onwards!) party last year, is now topping the polls and is forecast to beat far-right leader Marine Le Pen in an election run-off. The high number of undecided voters, however, means the ballot remains quite unpredictable.
On Saturday, Fillon's aides used an umbrella to shield him from eggs thrown by protesters in southwest France as the beleaguered conservative fell further behind Macron and Le Pen in opinion polls ahead of the April 23 first-round vote.
The UDI-UC has a total of 42 lawmakers in the French Senate.