<p>Hydrazine and hypochlorite are highly toxic and reactive compounds used in various industries and can pose serious hazards to human health and the environment, even at trace levels. Most of the reported methods for the detection of hydrazine and hypochlorite suffer from long response times, high operational costs, and limited on-site applicability, reducing their efficiency for practical and rapid monitoring. In this context, a phenothiazine–barbituric acid-based chemosensor (PB) was synthesized through a simple Knoevenagel condensation reaction and successfully used for the detection of hydrazine and hypochlorite. The probe showed a clear color change from deep violet to colorless, with strong fluorescence enhancement in the presence of hydrazine. In addition, PB responded to hypochlorite with noticeable color and fluorescence changes due to oxidative cleavage of the conjugated system. The probe exhibited high sensitivity, with detection limits of 0.89 nM for hydrazine and 0.94 µM for hypochlorite. The sensing mechanisms toward both analytes were supported by spectroscopic studies and DFT calculations, indicating nucleophilic cleavage for hydrazine and oxidative cleavage for hypochlorite. The practical applicability of the probe was demonstrated using test strips on real samples such as sand, rose petals, shrimp, and water. In all cases, rapid visual and fluorescence responses were observed under visible and UV light. These results indicate that PB is a simple, cost-effective, and efficient optical sensor for real-time detection of hydrazine and hypochlorite in environmental and biological systems.</p>