The 28-year-old Northern Ireland international returned to Wigan at the end of last season, following a loan spell at Inverness Caley Thistle.
McKay, who started his career at Leicester, joined Wigan from Caley in 2015.
The transfer is subject to international clearance, but County boss Jim McIntyre said: “I’m delighted to bring Billy to Dingwall.
“He is a player that we know well and the type of player that we feel can do well for us.
“I was keen to add more goal threat to our squad and Billy gives us that.
“He is great in and around the box and I am sure he will get goals for us.”