Quantum computing is an emerging force in sustainability, not because the technology is mature, but because its implications will arrive long before widespread deployment. There is a pressing near-term risk: today’s encrypted data is vulnerable to future quantum decryption, creating a “harvest now, decrypt later” threat that affects finance, trade, infrastructure, and any system dependent on long-term data integrity. At the same time, quantum computing holds transformative potential for sustainability, promising breakthroughs in materials discovery, energy optimisation, climate modelling, and industrial decarbonisation. It also carries significant environmental and ethical concerns, including extreme cooling requirements, energy use, and reliance on critical minerals. Quantum is already a strategic sustainability issue: its risks require early governance, and its benefits demand equitable access. Decisions taken now will shape whether quantum accelerates the green transition—or reinforces technological and economic inequalities.