As geopolitical instability increases and new threats emerge, a number of countries are increasing their respective allocations for defence expenditure in order to take greater responsibility for their citizens in terms of defending and protecting their values and way of life. Small states such as Lithuania, Latvia, and Estonia must evaluate certain economic, political, and strategic factors when increasing their respective defence expenditure. While they do tend to increase expenditure on national defence matters, budgetary constraints often force them to cut funding in some civilian domains or to increase their borrowing on international markets. Therefore, the security and defence of small states must be addressed in an integrated way, taking into account economic, social, and environmental factors. The aim of this article is to assess the relationships between defence expenditure and sustainable development indicators during the period between 2000 and 2018 in the Baltic states. The authors of this article aimed to determine which sustainable development indicators have a significant impact upon a country’s expenditure when it comes to defence matters. The study was conducted using econometric methods, including Spearman’s correlation analysis and Automatic Linear Modelling (ALM). The research results revealed some differences amongst the Baltic countries. In Lithuania, the employment rate and R&D personnel as a share of the active population demonstrated a significant impact upon defence expenditure. In Latvia, defence expenditure was found to be affected by disposable household income per capita and environmental taxes as a share of total tax revenue. In Estonia, defence expenditure was mainly influenced by disposable household income per capita and energy import dependency. The study’s findings may be used to ensure both the security of the country and the implementation of the Sustainable Development Goals.