The 23-year-old Dutchman arrived in Dingwall in October following his release from Go Ahead Eagles.
The former Netherlands U21 international has scored eight goals, including a last-minute winner in the League Cup final.
"We're delighted to get that over the line," County manager Jim McIntyre told BBC Scotland.
"Alex has done fantastically well since he's come to the club.
"He realises this has been good for him as well as it has for us. He's at a place where he feels he can grow and keep get better.
"He's got a coaching staff that really believes in him, a good group of team mates as well and he's happy within the club."
Contract talks lasted a number of weeks with County able to secure a deal, despite reported interest in the player from other clubs.
"There's always speculation when players are scoring goals, especially when he scores a winner in a cup final," added McIntyre.
"He had every right just to wait and see what offers were on the table. We just showed how much we wanted him here and we convinced him that this is the right place for him to be for the next year or so."
Schalk's retention is another example of McIntyre completing business early in advance of next season, as he did last term when recruiting a host of new players in an overhaul of the squad.
The former Dunfermline and Queen of the South manager concedes that there will be less activity over the summer.
"It'll certainly be a lot different than what it was last season when I was chasing umpteen players," he said.
"We've got a far more settled squad. There'll be additions but they won't be wholesale."
After achieving County's first major trophy in the League Cup and earning a top six place this term, McIntyre admitted his delight at being one of the nominees for PFA Scotland's Manager of the Year, to be announced on Sunday evening.
"Obviously, I'm very honoured that I've been nominated and it's just a reflection on how well everybody's done at the club," he said.
"There's some fantastic nominees and also ones that haven't been nominated have had great seasons also so I feel very privileged."
Ross County striker Alex Schalk has signed a new deal to remain with the club next season.