A fluorescein based reactive probe has been designed and synthesized to detect hydrazine selectively over other common analytes. We used here 4-bromobutyrate as a masking unit of fluorescein dye. Hydrazine plays here the role of a de-masking agent to set free the fluorescein dye through a simultaneous substitution–cyclisation–elimination process. This leads to ‘turn on’ fluorescence with easily discernible color change with a fast response time (<15 minutes).