Currently, the military actions on the territory of Ukraine require significant support from EU countries and partners in providing military and material assistance. The issue of openness and transparency of budgeting, particularly in the defence and security sector, becomes even more significant. The peak of interest in the literature on the issues of openness and transparency of budgeting appeared in 2005–2006. However, in Ukraine, which has largely continued to follow Soviet trends, this is an alarming subject. It has been brought to the forefront by the events after the full-scale invasion of the Russian Federation. One of the ways to guarantee the openness and transparency of budgeting is the development of a suitable open data system, which includes the analysis of all financial costs based on the proper methodology. Such a methodology should be founded on the concept of assessing the openness and transparency of budgeting and financial management of the defence and security sector of Ukraine at the current stage in the conditions of war and, after it, be measured quantitatively and implemented using IT. This article aims to consider the methodology of an index of openness and transparency of budgeting and financial management of the defence and security sector and to implement it in the case of Ukraine. Based on the conducted literature review, a new method to calculate the index of openness and budgeting transparency of the defence and security sector of Ukraine is built. Nine separate indicators are defined, and each of them affects the final value of the index. Some indicators have a binary form, and some have a scale, which is used to estimate their specific weight of impact. This approach makes it possible not only to monitor the openness and transparency of the defence and security sector but also to show the dynamics of the development of the phenomenon and compare it, in the future, with other countries. Based on calculations for 2008–2021, the trend of this index is shown for Ukraine, and conclusions are made regarding its further application.