But Mr Little would achieve "greater cohesion" within the party, he said.
"It has been an absolute privilege to lead the New Zealand Labour Party."
He said his family was "staunchly behind" his decision and he'd now be putting his full support behind Mr Little.
Mr Little said Mr Cunliffe had done the right thing by withdrawing.
"I think it's the right thing for the party and for himself, actually."
He said it had become clear Mr Cunliffe was unlikely to be able to make the changes the party needed to lift its support among the wider population.
Asked about Mr Cunliffe's endorsement of him, Mr Little said he appreciated any support he could get but it would still be a challenge to secure the leadership.
He repeated he was not offering positions to MPs before the leadership vote was held and had not offered Mr Cunliffe anything.
Despite that, Mr Cunliffe's endorsement could help Mr Little among the party's broader membership, especially in Mr Cunliffe's Auckland stronghold.
"I appreciate any support I can get. It's always welcome but I've still got my work cut out for me to convince those who are wondering if I've got enough experience in Parliament, and other qualities you need for the leadership. So nice as it is to have support, I don't underestimate the task ahead of me."
Mr Cunliffe secured a large proportion of the union vote last time, but much of that was expected to swing to Mr Little instead.
Acting leader David Parker ruled out having Mr Cunliffe as his deputy but would not rule out a front bench position. "That's a decision for then, not now."
Mr Cunliffe told him of his decision this morning. "He's made the right decision for him and the party, and he should be treated with the respect all former leaders deserve."
Asked about Mr Cunliffe's endorsement of Mr Little, Mr Parker said MPs could endorse whoever they liked. He did not believe it was a punishment for Mr Parker's statement he had lost confidence in Mr Cunliffe following the election. "But I can't answer for him and I won't."
He said it was Mr Cunliffe's right to decide to stay on in Parliament. "He's been elected and that's his right."
He was confident Mr Cunliffe would be able to settle into caucus again.
Read more:
• Claire Trevett: Labour's leadership contest is not about taking turns
• David Parker: Labour is 'a house divided'
• Editorial: It's time for Cunliffe to step aside
• Gallery: Cunliffe in cartoons
• Gallery: Cunliffe - Highs and lows
Mr Cunliffe's road to election night was marred by a series of gaffes.
The Labour leader caused outrage when he said he told a Women's Refuge Forum he was "Sorry for being a man", because men are usually responsible for violence against women and children.
While the crowd approved, the public were less forgiving and Labour's funding promise for victims' support groups was lost in the furore.
Mr Cunliffe then struggled to give clear answers on the effect of his party's flagship Capital Gains Tax on the family home, scaring mums and dads into believing a Labour-headed government could get a slice of what they saw as their children's birthright.
Following Labour's disastrous result on election night, Mr Cunliffe announced he would formally resign as leader but would enter the contest to win his job back - a decision that led to Labour's leadership primary run-off.
But support for his bid was thin.
EPMU, Labour's largest affiliated union, sent Mr Cunliffe a warning not to assume it would back him again if he forced another leadership run-off.
Meanwhile, wife Karen Price was unmasked for creating a fake Twitter account, set up to attack her husband's opposition.
Ms Price, an environmental lawyer, said she set up the Twitter handle @TarnBabe67 out of "extreme frustration" after the election and had used it to make comments that she now deeply regretted.
David Cunliffe
1999: Elected to Parliament in Titirangi seat after a career as diplomat and a consultant at Boston Consulting Group.
2002: Re-elected in new seat of New Lynn (previously Titirangi).
2003: Appointed Minister of State and Associate Minister of Revenue, Finance, Communications and ICT.
2004: Promoted to Minister of Communications, ICT and Associate Minister of State-Owned Enterprises. In the Communications role, he forces Telecom to end its monopoly by opening its local network to competitors, wiping $2.2b of the company's value.
2005: Holds New Lynn seat with a majority of 8000 votes. Appointed Minister of Immigration - where he leads major review of Immigration Act, introducing the seasonal worker scheme - and Associate Minister of Economic Development.
2006: Referred to Securities Commission for prematurely revealing Telecom's dividend plans but is later cleared.
2007: Promoted by Helen Clark to Minister of Health and ICT Minister.
2008: Re-elected in New Lynn but his majority is cut to 4000. After Labour's election defeat and Clark's resignation, he considers leadership bid but does not run and the job is taken by Phil Goff.
2011: Runs for leadership with Nanaia Mahuta as deputy but loses to David Shearer. Loses finance portfolio but picks up economic development.
2012: Amid speculation that he is considering a leadership challenge, he is demoted to the backbenches after refusing to pledge loyalty to Shearer at a Labour Party conference.
2013: Runs for the leadership after Shearer's resignation in the first contest to be decided by members and unions as well as caucus. After appealing to the party's unions and activist left, he wins the leadership race despite only winning support from a third of caucus. It is later revealed that he used a trust during the leadership contest to make sure his donors remained anonymous.
2014: Leads Labour to worst election defeat with just 25 per cent of the party vote. A week later he resigns but promises to run again in a new leadership contest. He is criticised for not taking responsibility for the defeat and is embarrassed when his wife Karen Price is found to have attacked his rivals with an anonymous online identity. A day before the nominations deadline, he pulls out of the leadership race and says he will remain in Parliament.
