This article analyses whether a rule of customary international law forbidding the exploitation of space resources has already been developed in the international community. The possibility of commercial exploitation of space resources rests in a grey area embedded in legal uncertainty, which might hinder further commercial activities in space. The panorama of applicable treaty law, notably the relationship between the provisions of the Outer Space Treaty and the Moon Treaty, has been subject to a comprehensive analysis of scholars. However, whether there is a customary law obligation forbidding the exploitation of space resources remains essentially unexplored. The answer to this question is especially relevant due to the relatively low number of ratifications of the Space Treaties and the absence of a specific framework for space resources exploration. We argue that there has not been yet formed a customary law rule forbidding the exploitation of space resources, since the requirements of state practice and opinio juris are not fulfilled. For the assessment of State practice, we consider the rare occasions where extraterrestrial matter was collected. A small amount of matter derived from extraterrestrial was collected and brought to Earth in several missions, but the quantity – very limited, and its purpose – scientific research, clearly distinguish these cases from the commercial exploitation of space resources which is currently intended. States have collected and retained ownership of extraterrestrial samples without opposition. Nevertheless, it is not possible to derive definitive conclusions from such a limited practice, especially in light of the fact that even the Moon Treaty allows the collection of samples for scientific purposes. Since State practice has been limited to the scientific collection of samples, we conclude that there is no sufficient State practice confirming a rule forbidding the exploitation of space resources. On the possibility of existing opinio juris on this issue, we discuss the hypothesis of the formation of customary law through the law-forming function of the applicable space treaties. The rule forbidding the appropriation of space resources might have been crystalized in customary law through the same path as the rule forbidding military use of space. The Moon Agreement and two UNGA Resolutions (unanimously approved) would provide a strong indication of the illegality of exploiting space resources. However, the exploitation of space resources seem to pose a different challenge than the rule forbidding military uses of the space. We find it difficult to find similarities in the rule forbidding military uses of the space, which was one of the deciding factors of the Space Treaties, with the totally unknown commercial uses of the space, such as extracting minerals from asteroids. The conduct of the US, and, to a lesser extent, of Luxembourg, of approving a law that clearly allows exploitation of space resources further indicates the absence of opinio juris in this regard. Hence, we argue that it would be highly contestable to affirm the existence of a customary international law rule prohibiting the appropriation based on current state practice and opinio juris.