Roberto Mancini previously already admitted Inter were keen to lure the 29-year-old away from Sampdoria, but claimed at the time a transfer in January could be problematic.
READ MORE | Official: Inter loan Ranocchia to Sampdoria
However, the San Siro side have now managed to capture the Italy international after all, with Eder signing a contract with Inter until June 2020, which will become valid once they exercise their option to buy.
"I am very happy to be here," Eder told the official Inter website.
"A lot of sacrifices and hard work have paid off and brought me at a big club like Inter."
Inter CEO Michael Bolingbroke, meanwhile, was quick to voice his delight with the transfer, too.
"We are pleased to have added Eder to our squad," he added.
"He is a player of great quality and with international experience. He will give our team a new dimension and will give Mancini another option."
Eder is the third piece of business between Inter and Sampdoria this month, with Andrea Ranocchia and Dodo previously moving in the opposite direction.