Mr Crozier built up ITV's production arm, reducing its reliance on shrinking advertising revenue.
The company, frequently tipped as a takeover target, said that a successor will be announced "in due course".
During the interim, ITV chairman Sir Peter Bazalgette will take on the role of executive chairman.
Finance director Ian Griffiths will lead the executive team until Mr Crozier's replacement is appointed.
Mr Bazalgette said in a statement that Mr Crozier "has made a fantastic contribution to ITV and the board is deeply indebted to him for his strong leadership and personal dedication in very successfully turning around the business and building a more global and diversified organisation with real scale in key creative markets around the world".
Mr Crozier, a former boss of the Football Association and Royal Mail, said: "Having spent 21 years as a chief executive across four very different industries, I now feel that the time is right for me to move to the next stage of my career and to build a portfolio of roles across the plc and private sectors."
ITV said the company's revenue from sources other than advertising had more than doubled to almost Â£1.9bn in 2016, more than half of its total, under Mr Crozier's tenure.
The broadcaster, which makes Coronation Street, is seen as a possible takeover target in an industry that is consolidating as viewers increasingly watch content on demand and on different platforms.
US media group Liberty Global, which owns 9.9% of ITV, had been tipped as a potential bidder, although it has previously said it did not want to buy the broadcaster.
ITV shares finished 1.75% lower on Wednesday.
ITV's chief executive Adam Crozier, who is credited with helping to turn around the broadcaster's fortunes, is to step down after seven years.