Celtic manager Ronny Deila insists Virgil van Dijk does not want to leave the Scottish Premiership leaders in January.
Celtic boss Ronny Deila says keeping his best players will take priority over acquisitions in the transfer window.
The 20-year-old right-back left The Dons in the summer after making 41 appearances for the Sky Bet League 2 club.
He said on his Twitter account: "Glad to sign for Ross County its a new challenge cant wait to start".
Fenlon left The Dons last month after rejecting a new deal.
Manager Neal Ardley told the club's official website at the time: "Jim had an option on his contract, which would have taken him up amongst the highest earners at the club. This was an option that we felt we could not take up considering his form and attitude during the past 12 months.
"As with all Football League contracts, the club has to offer at least the same terms as the player was on to retain compensation rights. However, with our budget that was a step too far for us.
"We offered Jim a slightly reduced offer in the hope that we would be able to add the attitude required to his ability. But we have not been given that chance and we therefore wish Jim all the best for the future."
Fenlon is County manager Derek Adams' eighth summer signing.