The aim of this paper is to analyze the concept of ‘non-appropriation’ in outer space from a legal point of view. The Outer Space Treaty in its Article II provides that outer space, including the Moon and other celestial bodies, is not subject to national appropriation by any means. In the absence of an official interpretation, the precise meaning of this provision has been discussed for decades. This paper will approach the problem by going back to the origin of the concept ‘appropriation’: more than 1500 years ago Roman lawyers had already developed different categories to indicate the relationship between a person and a thing. Ownership and use were the criteria utilized to classify each thing. Over time, these categories evolved and eventually led to the development of ‘property law’. Among them, the concept of res communes omnium was elaborated. Its peculiar features will be examined in this paper, underlining the economic function and the multifaceted nature specific of this category of things. Res communes omnium are emblematic of how ancient notions can find new life in the regulation of the cosmic dimension. Many of the legal problems faced by the international community today with regard to the cosmic environment are not different from the ones already faced by Roman lawyers when trying to regulate the reality around them. This paper will demonstrate how the ‘non-appropriation’ principle can be interpreted under the light of Roman legal theories. Building upon these findings, the legal status of outer space will be clarified and the scope of application of Article II of the Outer Space Treaty redefined. Roman theories on property rights can offer legal arguments for the use of space resources without breaching the Outer Space Treaty. Underlining the legal feasibility of commercial use of space resources as well as of settlements on other celestial bodies can hopefully represent an incentive for the international community to establish a regime regarding these activities. If that is not achieved, uncertainty will prevail and conflicts are certain to arise.