The former Liverpool and Paris Saint-Germain man has left Championship side Bolton for the injury-hit Welsh club, on a contract that runs until the end of the season
The 24-year-old, whose contract with the Championship club was due to expire at the end of the season, is Michael Laudrup's first signing of the transfer window and has signed a deal to the end of the current campaign.
The former France Under-21 international started his career at Paris Saint-Germain but was signed by Liverpool for £1.5 million in 2008.
N'Gog made 63 league appearances for the Merseyside club, but scored only nine goals and never established himself at Anfield.
He then joined Bolton Wanderers for a fee of £4m in August 2011 but suffered relegation to the Championship with the Lancashire side.
N'Gog scored eight goals in 31 league appearances in the second tier last season and has found the back of the net just three times in the current campaign.
The Frenchman will give Laudrup an extra attacking option in the absence of injured striker Michu, and Swansea are hoping N'Gog is given Premier League clearance to make his debut in Tuesday's clash with Fulham at the Liberty Stadium.