The governance of nuclear and other radioactive materials in “contested territories” has proved highly challenging, particularly in the former Soviet states, where numerous cases involving missing, lost, or stolen material took place in or near these uncontrolled territories. Yet, there are examples of successful cooperation between states and their breakaway regions, such as the Republic of Moldova’s removal of disused radioactive sources and material from the Transdniestria region. Since 2012, about 2,700 disused sealed radioactive sources and devices with radioactive materials have been removed, including orphan sources, from 24 sites/facilities in the Transdniestria region. The Organization of Cooperation and Security in Europe (OSCE) facilitated removal processes to a secure disposal and storage facility near Moldova’s capital Chisinau. Using data obtained from academic and technical literature, stakeholders involved in the removals, and technical experts, the paper analyzes political, technical, and other factors that contributed to the success of this initiative. It also derives the following lessons which others might be able to apply in tackling radiological security challenges in territories not controlled by the central government: (1) Start small with technical matters at a working level and then bring in leading political authorities on both sides of the conflict to sustain the effort; (2) Take advantage of the economic, security, and political/diplomatic incentives that each side may have to engage in cooperation in order to make the outcome a win-win for both sides; (3) Leverage pre-conflict professional relationships between experts on both sides to facilitate cooperation; (4) Secure financial assistance for removals of disused radioactive sources and materials from external sources, if possible, to avoid either side citing a lack of funding as a reason not to engage in the removal process; (5) Rely on international standards and the national regulator’s expertise in implementing cooperation; (6) Utilize a respected and independent international facilitator (e.g., OSCE) to provide both sides with confidence that the other side will hew to its commitments and limit politicization. It also helps if the facilitator embeds the work in radiological and nuclear security within a larger confidence-building process and has offices in both sides.