The 28-year-old French forward has signed a deal until the end of the season following his departure from Greek outfit Panionios.
N’Gog will team up once more with Owen Coyle, who brought him to Bolton Wanderers from Anfield for £4 million in 2011, where he scored 16 times in 91 games.
Coyle told Ross County’s website: “I’m delighted to bring a player of David’s calibre to Ross County Football Club.
“Having worked with him before at Bolton, I am all too familiar with his qualities and I think he will be an excellent addition to our attacking options.”
N’Gog, who netted 19 times for Liverpool in 94 matches was capped at Under-21 level for France and has also had spells with Swansea City, and Stade de Reims in his homeland.