<p>Astaxanthin, a natural red pigment belonging to the xanthophyll class, is widely found in microorganisms, algae, and crustaceans. This study addresses for the first time the challenge of efficiently extracting astaxanthin from shrimp by-products (<i>Aristeus antennatus</i>) using environmentally friendly GRAS solvents (acetone, ethanol, ethyl acetate, and isopropanol). For such a scope, astaxanthin content was identified and quantified by HPLC-DAD-APCI-MS, followed by the evaluation of the seasonal variation and the effect of thermal processing. Additionally, the antioxidant and antimicrobial potentials of the extracted astaxanthin were investigated to determine its functional value. Results demonstrated that acetone at a 1:20 solvent-to-solid ratio for 60 min at 30 °C achieved the highest extraction yield (1346.38 ± 37.31 µg/g). Astaxanthin content peaked in autumn and declined during summer, while cooking led to significant pigment loss. The extract exhibited strong antioxidant activity (EC₅₀ = 25.42 ± 1.43 µg/mL) and moderate antimicrobial effects (MIC range 25–1500 µg/mL). This study highlights the potential application of shrimp by-product-derived astaxanthin as a natural antioxidant and colorant in food industries, offering a sustainable valorization pathway for seafood waste.</p>