The 25-year-old made his debut for the Crusaders in 2013 following an outstanding season with the championship Canterbury team in the 2012 ITM Cup, in which he headed the top try scorers list with ten tries, three ahead of his nearest rival.
McNicholl can cover at full-back as well as wing and will link up with international stars DTH van der Merwe and Liam Williams to form a high-octane Guinness PRO12 back-line.
"This is a great opportunity to experience living and playing in a different part of the world," said McNicholl
"I am at a stage in my life that I would like to do some more travel while I don't have family commitments back in New Zealand, and I am so fortunate to be able to do that with my rugby career.
"The Scarlets club appeals because they are a successful team that plays a similar brand of rugby to the Crusaders, and working under a Kiwi coach in Wayne Pivac will make that transition easier."
Scarlets are currently gunning for a top-four finish in the Guinness PRO12 - they are three points ahead of fifth-place Ulster - and general manager Jon Daniels believes McNicholl is the type of signing that can bring long-term success.
"We are building a squad over time that can compete in the Guinness PRO12 and in the top flight of European competition, which means our recruitment and development strategies are stringent," he said.
"A player of Johnny's calibre will not only see us gain an experienced on-field player but will also aid us in the development of younger members of the squad going forward."
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