Rationally designed polydiacetylene (PDA) molecules have been developed for rapid, selective, sensitive, and convenient colorimetric detection of organophosphate (OP) nerve agents, a mass destruction weapon. Oxime (OX) functionality was incorporated into diacetylene molecules to utilize its strong affinity toward organophosphates. The diacetylene molecules having an OX functional group (OX-PDA) were self-assembled to form PDA liposomes in an aqueous solution. Upon exposure to organophosphate nerve agent simulants, OX at the OX-PDA liposome surface interacts with nerve agent simulants, which results in intraliposomal repulsive stress due to steric repulsion between OP-occupied OX units at the liposome surface as well as interliposomal aggregation induced by increased hydrophobicity of the liposome surface via OP-OX complex formation. The resulting intra- and interliposomal stress causes disturbance of the conjugated backbone of OX-PDA, producing color change as a label-free and sensitive sensory signal. The effects of molecular structure on selectivity and sensitivity of OX-PDA liposome solution, OX-PDA liposome-embedded agarose gels, and OX-PDA liposome-coated cellulose acetate membranes were systematically investigated. The optimized OX-PDA liposome in the solid state showed selective and rapid optical transition upon exposure down to 160 ppb of diisopropylfluorophosphate (DFP), a nerve agent simulant. The results provide an insightful molecular design principle of PDA-based colorimetric sensor and suggest portable sensory patches for rapid, selective, sensitive, and convenient colorimetric detection of organophosphate nerve agents.