In the role, the American billionaire will focus on mobilizing further climate action and commitments to net-zero emissions by 2050 from businesses and governments ahead of the U.N.'s Glasgow climate conference (dubbed the COP26) in November.
"Communities everywhere are feeling the worsening impacts of climate change," Bloomberg said in a video posted to his Twitter account on Friday. "Fortunately, more cities, states, businesses, and countries are taking climate action."
He said his new role is "to help them be bold and get results."
"As the world builds back from COVID-19, we have a once-in-a-lifetime opportunity to make investments that will strengthen the economy and improve public health and fight climate change for generations to come," Bloomberg added.
Bloomberg has previously been appointed to U.N. climate envoy positions in 2014 and 2018.
Guterres said the U.N. is pleased to welcome Bloomberg back during this "critical year."
"Mayor Bloomberg’s deep commitment to climate action and his essential experience bridging the interests of business, finance, policy makers and philanthropy are uniquely suited to helping to accelerate impact as we rebuild better together and drive toward a global commitment for COP26 and beyond," Guterres added in a statement.
Bloomberg's appointment comes just after the Biden administration entered the White House and immediately signaled that the U.S. was re-prioritizing climate action after the Trump administration backtracked on climate goals.