The 28-year-old, who plays wing or full back, is the fourth largest signing by the spot at a week later Wales internationals Jonathan Davies, Ken Owens and Rob Evans agreed to stay.
New Zealander McNicholl has scored 27 tries in 59 appearances since joining Scarlets at 2016 and can be eligible for a Wales on residency in December 20-19.
“I’m happy that everything has been finalised and I am staying,” he said.
McNicholl is going to be re-united with Brad Mooar, who takes at Scarlets when Wayne Pivac leaves to become trainer of Wales after in 20-19.
“We’ve had some fantastic success during the last couple of years and hopefully there is more in the future,” added McNicholl.
“I am looking forward to reconnecting with my former Crusaders trainer Brad Mooar and believe there are exciting times ahead.”
Pivac leaves his function at Parc y Scarlets at the summer before taking over from Warren Gatland as nationwide trainer Following World Cup from the autumn.
He said the Scarlets’ signings have been also a very clear statement of objective from the spot.
“He [McNicholl] has been perhaps probably one of the most dangerous attacking dangers in the Pro 14 in the past few years and played a significant role in people hitting consecutive finals and also a European semifinal,” said Pivac.
“With the headlines of Jon, Ken and Rob signing last week, this really is another significant registering up for the Scarlets. The team is still looking for next season”