Measures will be placed on finance for those nations to develop cleanly, cope with climate impacts and address the loss and damage they face from climate-related storms, floods, droughts and rising seas.
Climate financing for poorer nations more vulnerable to the worst impacts of climate change was a key issue at COP26, with developing nations also pushing for funding to cover loss and damage.
COP26 President Alok Sharma said around US$500 billion in financing for developing nations would be mobilised by 2025.
Wealthy nations in 2009 pledged to contribute US$100 billion per year to developing nations by 2020 - a deadline that was missed.
Tuvalu minister gives COP26 speech standing knee deep in the sea
Fiji's Prime Minister, Frank Bainimarama, said the COP26 negotiations had left Pacific Islanders feeling "battered, bruised".
"The 1.5-degree target leaves Glasgow battered, bruised, but alive," he said on Twitter.
"The compromise we’ve struck will only count if nations now deliver."
Fiji's Permanent Representative to the UN, Satyendra Prasad, said he would be relentless in working to "phase up" diplomatic efforts for the implementation of the 1.5C scenario.
"The hope of Paris and the security and stability of #BluePacific rests on 1.5," he said in a statement on Twitter.
"From tomorrow, we 'phase up' our climate diplomacy internationally and our actions domestically."
Greenpeace Australia Pacific’s COP26 Pacific delegate Shiva Gounden said the deal offered the Pacific Islands a glimmer of hope, but their survival depends on the world’s commitment to the agreement.
"This is the decade that will determine if our Pacific homes and way of life will continue, or if our most vulnerable will see their homes sink under the seas," she said.
Urgent pleas were made at COP26 by many Pacific nations who used the global stage as an opportunity to raise awareness of rising sea levels putting their people and homes at risk.
In an emotional address, Tuvalu's finance minister Seve Paeniu said that his country "is literally sinking".
"We are at the forefront of climate change. It is an existential threat now ... It is not fiction. It is not projected to happen in the future. Our land is fast disappearing," he said.
He said that the first two days of the conference showed promise, but low-lying countries like Tuvalu are desperate for immediate financial mechanisms to support climate mitigation efforts.
Pledges since the Paris Agreement in 2015 have bent the curve down towards less dangerous temperatures.
A series of other deals announced at COP26 have also shown promise; cutting methane, switching to electric cars, protecting forests, driving finance towards green tech, and a shift away from coal towards clean energy.
COP26 President Alok Sharma fought back tears as he said the final COP26 deal kept open the prospect of limiting global warming to 1.5C, but he added "its pulse is weak".
The COP27 summit in 2022, to be held in Egypt, will provide the next opportunity to check if the pledges contained in the COP26 deal are delivered.
“Countries must come to COP27 with plans consistent with the 1.5C goal that is the red-line for our region. Anything less will be another betrayal of the brave and resilient peoples of the Pacific,” Ms Gounden said.
Additional reporting by AAP