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Abstract BACKGROUNDHaematococcus pluvialis(Hp), a freshwater chlorophyte microalga, is a major natural source of astaxanthin (ASX), a potent antioxidant with anti‐inflammatory, anticarcinogenic and muscle pigmentation properties. However,ASXbioavailability is limited by the rigid cyst wall and, although cell wall rupture improves bioavailability, the free form is unstable under high temperatures,pHextremes, light or oxygen. Encapsulation techniques improveASXstability, making it suitable for functional foods and aquaculture, especially in salmonid feeds where natural pigments are preferred. The present study evaluates the stability of weakenedHp(Hp_w) biomass encapsulated in alginate (ALG) via ionic gelation. RESULTSEncapsulation utilizingALGachieved high efficiency (97 ± 2.63%) and loading capacity (32 ± 0.90%), confirming its suitability as a wall material.ALG‐Hp_whydrogels displayed significant color intensity, enhancing potential feed or food hues. Low bulk density (0.59 ± 0.01 g cm⁻³), moisture content (11.97 ± 0.20%) and water activity (0.28 ± 0.00) suggest minimized oxidation processes. Hydrogels measured 1.30 ± 0.06 mm with a uniform sphericity factor of 0.058 ± 0.03. Confocal laser scanning microscopy confirmed uniformHp_wdistribution andscanning electron microscopyrevealed fissure‐free surfaces, ensuring minimal permeability. DPPH (i.e. 2,2‐diphenyl‐1‐picrylhydrazyl) scavenging activity was similar betweenHp_wextract (38.32 ± 2.30% to 96.32 ± 0.88%) andALG‐Hp_whydrogels (33.20 ± 1.55% to 93.30 ± 0.44%).ALGIncreasedHp_wdecomposition temperature by 40.97 °C. Encapsulation ofHp_winALGsignificantly enhanced the bioaccessibility ofASX. TheALG‐based encapsulation effectively preservedASXstability, retaining over 90% of its content under storage conditions. CONCLUSIONALGis a suitable biopolymer for encapsulatingHp_w, preserving antioxidant activity, and enhancing thermal properties, making it valuable for broader applications. © 2025 Society of Chemical Industry.