We present a unified many-body perturbation theory for open quantum systems that treats dissipation, correlations, and external driving on equal footing. Using a Keldysh-Lindblad formalism, we introduce a diagrammatic treatment of dissipative interaction lines representing quasiparticle flows and fluctuations. Two new Feynman rules render the evaluation of dissipative diagrams compact and systematically improvable, while preserving the Keldysh and anti-Hermitian symmetries of the closed-system theory. Consequently, the structure of the Kadanoff-Baym equations remains unchanged, enabling existing numerical methods to be directly applied. To illustrate this, we derive dissipative versions of the second Born and $GW$ approximations and identify the physical content of the self-energy components. Moreover, we demonstrate that time-linear approximations to the full Kadanoff-Baym equations retain their closed-form structure and can be efficiently used to simulate relaxation and decoherence dynamics. The impact of dissipation-induced correlations is illustrated in the driven Haldane model, where quasiparticles exhibit nontrivial stabilization and acquire lifetimes that far exceed those of the bare system. This framework establishes a general route toward first-principles modeling of correlated, driven, and dissipative quantum materials.

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• Open quantum systems
• Diagrammatic methods
• Feynman diagrams
• GW method
• Nonequilibrium Green's function
• Perturbation theory