UN secretary-general António Guterres described the deal as a "breakthrough" after nearly two decades of talks which began in 2004.
"This action is a victory for multilateralism and for global efforts to counter the destructive trends facing ocean health, now and for generations to come," he said.
Referred to as the 'High Seas Treaty', the legal framework would place 30 per cent of the world's oceans into protected areas, put more money into marine conservation, and covers the use of marine genetic resources.
The treaty is considered to be crucial for addressing crises including climate change, biodiversity loss and pollution.
It is an update on the UN's Convention on the Law of the Sea, which was adopted in 1982, and laid down rules governing the use of the oceans and their resources.
"It is also vital for achieving ocean-related goals and targets of the 2030 Agenda for Sustainable Development, and the Kunming-Montreal Global Biodiversity Framework," the UN said in a statement.
Many countries have already signed up to the '30x30' target, which is aiming to protect at least 30 per cent of the world's land and ocean habitats by 2030. In December, the UK pledged £34m to preserve wildlife, plants and habitats across the globe as part of the efforts.
Boris Johnson is among the UK politicians and campaign groups welcoming the historic agreement.
Speaking on Twitter, he said: "This is wonderful news for the world. It's time to stop the plunder and the pollution of our seas. I am proud of the way the UK has helped to lead."
Foreign Office minister Lord Goldsmith of Richmond Park tweeted: "This really is a historic moment.
"The ocean covers two-thirds of the planet, provides half the oxygen we breathe, absorbs vast amounts of CO2 & provides food for billions. This treaty will mean much greater protection."
The UN statement also recognised the "critical support" of non-governmental organisations, civil society, academic institutions and the scientific community.
Guterres "looks forward to continuing working with all parties to secure a healthier, more resilient, and more productive ocean, benefiting current and future generations," the statement concluded.
A 2020 study found that depleted marine life across the world's oceans could recover to healthy levels in just 30 years if major threats such as climate change are dealt with.
