The pledge sees the banking giant build on its previous 2020-21 goal for £20bn of climate and sustainable funding and financing, which was achieved six months early.
It comes as a report by NatWest revealed a £160bn revenue opportunity for small firms as a result of the green drive, which could see about 130,000 jobs created in the sector and some 30,000 new companies.
The bank, which remains majority owned by the taxpayer, said the sustainable funding pot will be used to support all customers, including small businesses, as well as to invest further in areas such as renewables, green mortgage deals for homeowners and electric vehicles.
Chief executive Alison Rose said: "We have identified the potential opportunities as being worth £160bn for SMEs [small and medium-sized enterprises] and the UK economy, and we want to do everything possible to support our customers in achieving a share of that prize.
“Our ambition to play a leading role in the UK’s transition to a net zero economy is why we are targeting £100bn of climate and sustainable funding and financing by the end of 2025.
“I’m firmly of the view that we should never underestimate the power of the small - and in this instance SMEs - for leveraging big advances.”
The bank also plans to launch new tools for businesses to monitor their carbon footprint and provide climate training for all its relationship managers in collaboration with the University of Cambridge and University of Edinburgh.