Sanofi shares fell 3.4 percent by 0920 GMT, making the stock the worst performer on France's benchmark CAC-40 index and several analysts deemed the deal expensive.
The move comes at a time of renewed interest by large drugmakers in smaller biotech firms and predictions by some experts that 2018 will see a substantial pick-up in mergers and acquisitions.
Sanofi has agreed to buy all of the outstanding shares of Bioverativ for $105 per share in cash, marking a premium of 64 percent to Bioverativ's closing price on January 19.
Bioverativ, a maker of haemophilia drugs, was separated from Biogen Inc early last year.
The agreed transaction marks Sanofi's successful return to deal-making after its failure to land major takeovers in recent years. It is its biggest acquisition since the 2011 takeover of U.S. biotech company Genzyme for around $20 billion.
Sanofi lost out on buying California-based cancer specialist Medivation to Pfizer in 2016, and also missed acquiring Swiss biotech company Actelion, which was bought by Johnson & Johnson last year.
"With Bioverativ, we welcome a leader in the growing haemophilia market," Sanofi Chief Executive Olivier Brandicourt said.
The market dealing with treatments for haemophilia is an important one that is evolving rapidly as new drugs change the landscape. Further ahead, gene therapy promises to disrupt traditional approaches to tackling the inherited condition.
Sanofi said the sector had around $10 billion in annual sales, dealing with 181,000 people affected worldwide. It added that haemophilia represented the largest market for rare diseases and was set to grow by more than 7 percent per year through to 2022.
Sanofi expects the acquisition to be immediately accretive to its business earnings per share in the full 2018 financial year and up to 5 percent accretive for the following year.
However, some analysts questioned the cost.
"Bioverativ looks a relatively expensive acquisition. It is logical in terms of building around Sanofi's presence and pipeline in rare diseases and haemophilia, though management may have to argue against concerns on competition," analysts with Jefferies wrote in a note to clients.
"The obvious parallel is Shire's highly unpopular acquisition of Baxalta; but Sanofi's 2011 acquisition of rare disease specialist Genzyme was also unpopular at the time, yet has it turned into a major success story," analysts with Kepler said.
Sanofi added it would fund the takeover with a mixture of existing cash resources and a debt issue.
"We have the means to make further takeovers," added Brandicourt on a conference call, without going into further details.
Brandicourt has said in the past he was ready to do deals of a similar size to the $20 billion purchase of Genzyme to help accelerate growth at France's biggest drugmaker. One asset analysts and bankers believe could be of interest is Pfizer's consumer health unit, although competition for this business is likely to be fierce.
Sanofi said it expected to achieve a return on its invested capital (ROIC) in excess of the cost of capital within three years. The French group also expects to preserve its strong credit rating.
This month Celgene agreed to pay up to $7 billion to take over Impact Biomedicines and the U.S. company is also said to be circling Juno Therapeutics.
In Europe, Novo Nordisk has offered $3.1 billion for Ablynx and Japan's Takeda Pharmaceutical plans to buy TiGenix for $630 million.
The spate of deal-making follows a relatively subdued 2017 for biotech M&A.
Lazard advised Sanofi on the deal, while Guggenheim Securities and J.P. Morgan advised Bioverativ.
(Additional reporting by Ben Hirschler in London, Laurence Frost and Matthias Blamont in Paris, and Shubham Kalia in Bengaluru; editing by Jason Neely and Keith Weir)
By Sudip Kar-Gupta