Managing director of ITV Studios Kevin Lygo will replace him next month.
Fincham said it was "time to do something different" but added that while he had "no firm plans", he would consider a return to the indie sector.
ITV's chief executive Adam Crozier paid tribute to Fincham as a "great creative partner" and added that he was "delighted" by Lygo's appointment.
Julian Bellamy, managing director of ITV Studios in the UK, will succeed Lygo.
Lygo and Bellamy start their new roles on 1 February, with Fincham staying until the end of March to complete the process of handing over the role.
Fincham, former controller of BBC One, said: "This is a personal decision that I have been thinking about for a good while.
"By the time I leave at Easter I will have been a channel controller at the BBC and ITV, amazing as it seems, for 11 years. Quite simply, it's time to do something different."
He said he had worked at ITV "during a period of great revival", and added: "I haven't made any firm plans for the future yet, but I wouldn't rule out a return to the indie sector - I ran TalkBack for a lot longer than I've been on the broadcasting side and that indie heart has never quite stopped beating. "
His commissions for ITV include Downton Abbey, Broadchurch, Long Lost Family and The Only Way is Essex.
Crozier said: "Peter came to see me in September to say that he wanted to step down and whilst I am sorry to see him leave, eight years is a long time and I fully respect his desire for a new challenge."
He added Lygo was "perfectly placed to lead the commissioning team as we enter this next exciting phase".
Lygo - Channel 4's former director of television - said: "I've had a terrific time over the last five years building ITV Studios in the UK and internationally into the success story that it is today, but the chance to become director of television of the UK's biggest commercial broadcaster was just too good to miss."
ITV also announced David McGraynor, ITV Studios finance director, was to become the chief finance and operating officer of the studios division.

Christophe Hillairet expressed concern that food produced outside the European Union could easily cross a "soft" Irish border and enter EU markets.
He is the president of the Chamber of Agriculture of Ile de France.
However, his comments were criticised as "disappointingly self-centred" by an Irish dairy industry leader.
Speaking to the Agra Europe website, [Â£] Mr Hillairet said he was worried that the British government will sign deals to import food from Commonwealth countries post-Brexit and that those products could then be transported across a future EU land border via Northern Ireland.
"Ireland is a big problem but for the French farmer, we will need to have a hard border between the north and the Republic, as otherwise we will have a lot of products that will cross from north to south.
"That would be very dangerous for our producers."
Both the British and Irish governments have repeatedly said they do not want a return to a hard border on the island of Ireland, expressing concerns over the potential impact on the economy and the Northern Ireland peace process.
However, the EU's chief Brexit negotiator, Michel Barnier, has said that there will have to be some form of customs controls along the Irish border after Brexit.
On a two-day visit to the Republic of Ireland last week, Mr Barnier said:  "We want to find solutions without rebuilding any kind of hard border, but we have to find solutions also compatible with the single market."
Mr Hillairet has been president of the Chamber of Agriculture of Ile de France since 2006.
He told Agra Europe that he believed it was "simply not possible to have a soft Brexit and still maintain the advantage which the EU has a trading bloc".
He urged remaining EU member states to "work together to protect Europe".
Mr Hillairet's remarks were criticised by the president of the Irish Creamery Milk Suppliers Association (ICMSA), John Comer.
He argued that the interests of Irish border communities, who had suffered during Northern Ireland's Trouble, must also be protected.
Mr Comer said that "with respect to our French friends, we would prefer them to reflect on the common good and not just on their own particular sectoral anxieties".
ITV director of television Peter Fincham is leaving the post after eight years, it has been announced.
[NEXT_CONCEPT]
The president of a French farming union has called for the re-introduction of a hard border between Northern Ireland and Republic of Ireland after Brexit.