Introduction: Extreme heat increasingly threatens older adults, particularly those with chronic conditions, yet generic heat-health advice may not be sufficiently timely or relevant to individual needs. This feasibility study describes a prototype and assesses the feasibility of a location-triggered, disease-specific heatwave short message service (SMS) intervention tailored to common heat-vulnerability conditions, compared with generic heatwave SMS advice. Methods: Mixed-methods feasibility study comprising a parallel two-arm 1:1 randomised controlled trial and post-heatwave focus groups. Community-dwelling Australians aged [≥]65 years in New South Wales, Victoria or South Australia with at least one eligible chronic condition (cardiovascular diseases, respiratory conditions, diabetes, and chronic kidney diseases) and a smartphone were recruited in summer 2026. Based on an initial codesign, participants received a 'prepare' SMS after enrolment and, when Bureau of Meteorology heatwave warnings were triggered, messages before, during and after heatwaves. Control participants received generic 'standard care' heat-health advice; intervention participants received condition-tailored messages and could request additional information via SMS codes. Outcomes were collected via baseline and post-heatwave surveys and thematic analysis of focus groups. Results: Seventy-three participants enrolled (36 control; 37 intervention); attrition was 9.6%. Intervention engagement was strong: 61% requested additional information, with frequent free-text replies and multi-condition requests indicating preference for more conversational interaction. Eight participants were heatwave-exposed and completed post-heatwave surveys (4 per arm), with a high usability score (median of 85/100). Among these 8 participants, 7 reported adopting heat-protective health behaviours; the most common were drinking more water (6/7). More total actions were reported in the intervention group (11 vs 8). No adverse effects were reported. Conclusion: A location-triggered, disease-tailored heatwave SMS system for older adults with chronic conditions was feasible, acceptable and highly usable, with high engagement and no harms. Findings support a larger trial and suggest benefits from tailored messaging.