We calculate the lifetime of the deuteron with dimension-nine quark operators that violate baryon number by two units. We construct an effective field theory for |ΔB|=2 interactions that give rise to neutron-antineutron (n-n[over ¯]) oscillations and dinucleon decay within a consistent power counting. We calculate the ratio of the deuteron lifetime to the square of the n-n[over ¯] oscillation time up to next-to-leading order. Our result, which is analytical and has a quantified uncertainty, is smaller by a factor ≃2.5 than earlier estimates based on nuclear models, which impacts the indirect bound on the n-n[over ¯] oscillation time and future experiments. We discuss how combined measurements of n-n[over ¯] oscillations and deuteron decay can help to identify the sources of baryon-number violation.