"Signing is good, ratifying is better," Hollande quipped at the Elysee Palace ceremony, flanked by Environment Minister Segolene Royal, Foreign Minister Jean-Marc Ayrault and other top officials.
He noted that the deal will not come into force unless at least 55 countries responsible for at least 55 percent of global greenhouse gas emissions ratify it.
So far 17 states -- mainly small island and low-lying coastal countries that are especially vulnerable to the sea-level rise -- have ratified the deal.
Hollande called on other European countries to follow France's lead by the end of the year.
The United States, the biggest emitter of greenhouse gases after China, will ratify the accord soon, US Secretary of State John Kerry said Wednesday during a visit to Oslo.
Norway's ratification is also imminent following a green light by its parliament on Tuesday.
"The United States will join soon, this year and together we are going to work to bring this agreement into force as quickly as possible," Kerry told a news conference in Oslo.
Kerry was to overfly the Norwegian archipelago of Svalbard, north of the Arctic Circle, on Thursday "to see first hand the impacts of climate change on the Arctic".
At the December COP21 gathering in Paris, 195 governments agreed to a target of limiting global warming to "well below" 2.0 degrees Celsius (3.6 Fahrenheit) compared with pre-industrial levels.
To achieve even the two-degree target, additional investment of 4.7 trillion euros) in zero-carbon power -- on top of an already projected 7 trillion -- would be needed by 2040, a key energy report said Monday.
The French hosts of the meeting, held just weeks after the devastating November terror attacks on Paris, were showered with praise for its success, notably Hollande and then foreign minister Laurent Fabius.
The 32-page deal also calls on rich nations to muster at least $100 billion a year in climate aid from 2020. Just how that will happen has yet to be worked out.
COP21 is the acronym for the 21st conference of parties to the UN Framework Convention on Climate Change (UNFCCC), the arena set up under the 1992 Earth Summit in Rio.