The 30-year-old was previously with Wigry Suwalki, who are 14th in Poland's second tier.
County manager Jim McIntyre said: "He has 35 international caps for Lithuania under his belt, bringing a wealth of experience to the squad."
Sernas started his career with Dainava Alytus in his homeland and has also played in Russia, Turkey and Australia.
After leaving Dainava, he had spells with Vttra, Spartak Nalchik, Widzew Lodz, Zaglebie Lubin and Gaziantepspor, who sent him on loan to A-League outfit Perth Glory last year.
County have already signed former Celtic defender Marcus Fraser,striker Craig Curran, previously of Nuneaton Town, and defender Cameron Burgess on loan from Fulham during January.
The Dingwall side are bottom of the Scottish Premiership, four points behind 11th-placed St Mirren but with two games in hand.

LeMond, 52, is the only American winner of the Tour after Armstrong was stripped of his seven titles in 2012 for systematic doping.
"If he was clean and everybody else was clean, he's a top-30 [Tour finisher] at best," LeMond told CNN.
"He's not capable of winning the Tour, he's not capable of the top five."
Armstrong, 42, was also banned for life following a United States Anti-Doping Agency investigation into the doping practices of the United States Postal Service team, a punishment LeMond feels is fair.
"There are people who are criminals who just shouldn't be able to participate again in anything," he said.
"It's like [jailed financier] Bernie Madoff - should he be allowed to come in and be a part of Wall Street managing money? No, he shouldn't.
"And that's [the case] with Armstrong. He shouldn't be allowed to be back in sport."

The caves have been digitally reproduced as part of the city's first Festival of Caves.
The tour will take in Mortimer's Hole under Nottingham Castle and King David's Dungeon - caves some people would not be able to access.
The city has more than 500 man-made sandstone caves which have been used for dwellings and jail cells.
Nottingham City councillor, Dave Trimble, said: "There are plenty of local people who don't know about the hidden world beneath their feet."
He said the new virtual reality tour would allow visitors who were unable to descend the steep steps into the caves to experience them.
Andrew Whitney of Hot Knife Digital Media, which developed the tour, said: "We've taken very accurate data to digitally reproduce the caves in a gaming environment.
"We're excited to bring the caves into the 21st Century using the latest gaming and virtual reality technology, increasing the accessibility of these sites to everyone."
A series of caves has been found in the past six years due to funded projects such the Nottingham Caves Survey, which mapped and laser-scanned many of them.
The festival runs until 23 October.
Lithuania midfielder Darvydas Sernas has become Ross County's fourth signing of the January transfer window.
[NEXT_CONCEPT]
Three-time winner Greg LeMond believes Lance Armstrong was only capable of a top-30 Tour de France finish "at best" without performance-enhancing drugs.
[NEXT_CONCEPT]
Nottingham's network of caves has been turned into a virtual reality tour, to open them up to a wider audience.