This Declaration on Funding Research Software Sustainability builds on actions undertaken by the Research Software Alliance (ReSA), research funding organisations, and the community surrounding it to develop awareness about the role funders can play in sustaining software in the longer term. It was initiated during an international workshop attended by representatives of funding organisations in Amsterdam on 8-9 November 2022, organised by ReSA and the Netherlands eScience Center. The Declaration is a first step towards formalising, on a global level, the basic principles and recommendations related to funding the sustainability of research software, including the people needed to achieve this goal. The aim of this Declaration is to raise awareness of the role of funding practice in the sustainability of research software, and to improve that practice. The Declaration includes a limited number of recommendations and an accompanying ADORE.software Toolkit. For the purposes of this declaration, research software is defined as “all forms of software that were created during the research process or for a research purpose”. A fuller description is included in the accompanying ADORE.software Toolkit