The Outer Space Treaty enshrines a core tenet of international space law: outer space is the province of all humankind and its exploration and use shall be carried out for the benefit and in the interests of all countries. It prohibits specific types of military activity – including the placement in orbit around the Earth any objects carrying nuclear weapons, the stationing of such weapons in outer space, and the installation of military bases on celestial bodies – but is silent as to the placement and use of other, more conventional weapons and the militarisation of outer space more broadly. Since the Cold War, that silence has been harnessed by spacefaring nations to develop military technology for use in outer space, such as military satellites and the Global Positioning System. In the last two decades, spacefaring nations have used outer space to test conventional military weaponry – in some instances by destroying their space objects, thereby creating space debris in the result. The militarisation of outer space has the potential to threaten its sustainability if weapons are used for offensive purposes. The creation of space debris, for example, may pose a threat to valuable assets (civilian and military) and the environment. This paper will explore the ways in which outer space has been militarised since the adoption of the Outer Space Treaty and assess the extent to which militarisation has had (and may, in the future, have) ramifications for the long-term sustainability of outer space. Can the Outer Space Treaty accommodate the militarisation of outer space? If so, why and to what extent? If not, what steps can be taken to ensure that the interests of spacefaring nations are preserved while at the same time preventing catastrophe – environmental and otherwise – in outer space? This paper will endeavour to answer these large questions.