High-temperature stress significantly threatens the aquaculture production of Kappaphycus alvarezii. Physiological and biochemical responses of red and green strains exposed to a temperature gradient (20oC to 36 oC) over 15 days. Both strains thrived optimally at 20 oC -24 oC. However, temperatures > 28 oC triggered reduced growth, severe whitening, and degraded photosynthetic pigments, with the red strain exhibiting a drastic growth decline of over 80% at 32 oC and greater photosystem II damage. Conversely, the green strain demonstrated superior thermal tolerance, maintaining higher pigment stability and greater morpho-structural integrity. Under thermal stress, the green strain exhibited enhanced reactive oxygen species (ROS) scavenging via elevated antioxidant enzyme activities (SOD, CAT, APX) alongside robust osmo-protectant accumulation (proline and soluble sugars). These findings highlight the green strain’s resilient protective mechanisms, offering valuable selection insights for climate-resilient seaweed cultivation.