We explore nonequilibrium topological responses of symmetry-protected topological (SPT) states in open quantum systems subject to decoherence. For SPT wave functions protected by a product symmetry $G\times S$, where $G$ defects are decorated with $S$ charge, we show that full dephasing of the $S$ charge density induces spontaneous strong-to-weak symmetry breaking (SWSSB) of $G$ in the resulting mixed-state ensemble. We extend this mechanism to SPT phases protected by higher-form and spatially modulated symmetries, and further to gapless SPT states, demonstrating that dephasing-induced SWSSB persists well beyond conventional gapped zero-form settings. Our results provide a qualitative, channel-defined fingerprint of SPT order that is intrinsic to open-system dynamics and goes beyond equilibrium linear response.

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• Open quantum systems & decoherence
• Symmetry protected topological states
• Lindblad equation