He will be replaced by customer director Jolie Hodson, who has been with the telco since 2013.
Moutter has recently driven an "agile" restructure of Spark, plus an aggressive drive into sports content and streaming.
The Spark veteran had given the market no indication he was thinking about leaving.
Hodson was named Deloitte CFO of the Year in 2016. The same year, she told the Herald she wanted to break through the glass ceiling and become CEO one day.
Moutter served as chief operating officer in the Theresa Gattung era before leaving to become chief executive of Auckland Airport. He returned to Spark in 2012 to take the top job.
This morning, Spark chairwoman Justine Smyth said when Moutter became Managing Director in August 2012, he had done so in the expectation of a likely five-to-seven-year tenure.
When he became MD in 2012, Moutter was seen by some commentators as a fix-it technocrat who would focus on restructuring the telco's technology, organisation and staffing. He did that, but also initiated a bold move into content, first with Lightbox then Spark Sport.
Just how bold is still not known. Although the Herald has been told by an insider at free-to-air partner TVNZ that Spark Sport is running over budget, that budget has not been made public.
Moutter has said on several occasions that Spark won't reveal the cost of acquiring and streaming sports content until its first financial results after the Rugby World Cup.
