The former Paris Saint-Germain and Monaco striker was a free agent after his contract with Al-Gharafa expired in January, and manager Sam Allardyce has acted quickly to sign the 33-year-old.
Nene, who scored 36 goals in 79 league appearances for PSG, could go straight into West Ham's squad for their Premier League clash with Tottenham on Sunday.
Carroll was ruled out for the season last week after suffering his latest injury setback in West Ham's goalless draw with Southampton on February 11.
With Carlton Cole, Enner Valencia and Diafra Sakho all fit, West Ham now have a four-man strikeforce to finish a promising campaign on a high.
And an elated Nene is relishing the chance to help his new club succeed in the Premier League.
"I am very excited and happy to be here," the Brazilian told the club's official website. "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."
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