Abstract: Current debates on the prevention of an arms race in outer space are dominated by the perspective of international strategic balancing. This article addresses the issue through the often-neglected lens of environmental protection. It is argued that environmental protection has become an increasingly relevant consideration for negotiating arms control more for its instrumental value to other interests, rather than the intrinsic value of the environment. Although the existing regime of outer space law has pacified part of the environmental concerns, in particular that arising from nuclear contamination, it fails to address the issue of proliferation of space debris which currently is the major impediment to the sustainable use of outer space. Customary principles of environmental law, in particular the no-harm principle and the principle of sustainable development, only constrain the way military activities are conducted in outer space, rather than the right to conduct them per se. It is further advocated that arms control in outer space should be strengthened for the common interests of mankind derived from a sustainable space environment. Such a treaty should at least prohibit testing, deployment and use of space-based weapons and Anti-Satellites Weapons.