The two companies "announced that they have entered into a definitive agreement whereby Johnson & Johnson will acquire Synthes for 159 Swiss francs per share, or $21.3 billion," (15.7 billion euros) they said in a joint statement released on Synthes's website.
"The transaction is expected to close during the first half of 2012," the companies added.
Although the deal has been approved by the boards of Synthes and Johnson & Johnson, it is still subject to approval by shareholders as well as regulatory authorities in the United States and European Union.
Such a transaction could boost the US giant's medical equipment business, which made up 40 percent of its 2010 revenues.
Synthes specialises in instruments, implants and biomaterials for use in orthopaedic surgery.