Air defence systems, munitions and drones were also included in the letter of intent signed by Ukrainian President Volodymyr Zelenskiy and his French counterpart Emmanuel Macron in front of one of the jets and both their flags on Monday.
"It will be the greatest air defence, one of the greatest in the world," Zelenskiy told reporters at the event at France's Villacoublay military airport.
Shares in Dassault, which makes the jets, rose sharply on the news and were eight per cent higher within hours.
The announcement comes after a surge of Russian drone and missile attacks on Ukraine in recent weeks, and Moscow's reports of ground advances in the southeastern Zaporizhzhia region.
The letter of intent was a political commitment, rather than a purchase deal, which would come later, the Elysee said. The aim was to finance it with EU programs and the planned use of frozen Russian assets, which the EU still has to agree.
It covered new material rather than transfers from French stocks, the Elysee added.
"We're planning Rafales, 100 Rafales - that's huge. That's what's needed for the regeneration of the Ukrainian military," Macron told LCI TV.
The aim was to help Ukraine in the short term with drones, drone interceptors and other gear - and also in the longer term to allow it to fight off any new incursion once there is a peace deal, he added.
The deal includes the acquisition of the new generation of the SAMP/T air-defence batteries, which is currently being developed, and AASM Hammer air-to-surface munitions, both sides said.
There have been talks for several weeks to see how France could provide more military support for Ukraine's air defences, though there have been questions about how much Macron's government will be able to do as it grapples with political and budgetary instability.
France, along with Britain, has pushed for the creation of a coalition of about 30 countries willing to send troops and assets to Ukraine or along its western borders once a peace deal with Russia is agreed.
Operating the advanced Rafale jets would take time given the rigorous training program for pilots.
