The Hammers moved quickly to replace striker Andy Carroll who has been ruled out for the rest of the season with a knee injury.
The 33-year-old Nene played for French Ligue 1 clubs Monaco and Paris St German before joining Qatari side Al-Gharafa in 2013.
"I'm proud to come to a really big club like West Ham. I've been thinking about coming back to Europe for a few months because here, the football, the ambience, everything is better," Nene told the West Ham website.
"I like to get on the ball and show my technique," he added. "I like taking free kicks and I like making assists."
West Ham are eighth in the standings with an outside chance of challenging for a top-four finish and a Champions League qualification spot with 13 games left in the season.
(Reporting by Ed Osmond; Editing by Frank Pingue)
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