Many studies have demonstrated the prominent role legal frameworks and local policies play in the shaping of prostitution, by informing to a large extent the conditions governing the exercise of the sex trade, while promoting a certain definition of this activity and its protagonists. However, the role of private organizations delegated the mission of providing social or medical assistance to people selling sex should not be overlooked. These organizations are still under-researched, despite the fact that they often occupy a pivotal position between those involved in the sex trade, public authorities, and the general population. Our contribution aims to provide an overview of the relevant landscape of third sector organizations in both Belgium and France and, more specifically, retrace the genesis of associations that have implemented programs to prevent sexually transmitted infections. We will also examine their relations with the public authorities and the legitimacy they enjoy in each country, before highlighting their potential influence on the structuring of representations and regulation of prostitution.