Back in 2011, Coyle signed N'Gog for Bolton from Liverpool for £4m.
Seven years later, the Scot has brought his man to Scotland on a free transfer after the Frenchman recently left Greek Superleague outfit Panionios. Before that he was at Reims for two years, after a fruitless spell with Swansea.
N'Gog will be best remembered by Liverpool fans as a regular substitute who played in most of their Europa League clashes.
When the forward was with the Merseyside club between 2008 and 2011, he made a total of 94 appearances - 53 of which were off the bench - and scored 19 times.
Now aged 28, N'Gog is looking to revive his nomadic career in Scotland and Coyle is proud to secure his services again.
"I'm delighted to bring a player of David's calibre to Ross County Football Club," Coyle told the club's official website.
"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."
Ross County are bottom of the Scottish Premiership table in 12th.
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