The 33-year-old Brazilian was available on a free transfer after leaving Qatari side Al-Gharafa last month.
The signing will boost manager Sam Allardyce's attacking options following the loss of frontline striker Andy Carroll to long-term injury.
Nene told the club's website, www.whufc.com: "I am very excited and happy to be here
I'm proud to come to a really big club like West Ham and I'm going to do my best to help the team.
"I've been thinking about coming back to Europe for a few months because here the football, the ambience, everything is better.
"It's a really good challenge for me, to come to a competition like the Premier League
It's the most competitive in the world so this is the motivation for me."
The much-travelled Nene, whose previous clubs also include Mallorca, Alaves, Celta Vigo and Monaco, was joint-top scorer in Ligue 1 after scoring 21 goals for PSG in the 2011-12 season.
Nene offers verstaility as he can play out wide while he also has a reputation as a dead-ball specialist.
He said: "I like taking free-kicks, so practise to be good at that
I like making assists - that is my real work, to make the final pass to put the striker in a good position to score the goals."
Carroll, 26, is expected to miss the rest of the season after undergoing knee surgery on Tuesday.
The former Newcastle and Liverpool forward suffered ligament damage in last week's goalless Barclays Premier League draw at Southampton.
It is yet another major blow for the England international after missing most of last season with an Achilles problem.
Source : PA
Source: PA