Gracilaria changii is a red seaweed macroalgae that holds economic potential for the Malaysian economy, as it has now started to be cultivated commercially. Establishing the harvesting standards for seaweed is important for its marketing purposes. Thus, this study evaluated the effect of different growth stages on its postharvest quality. The seaweed was studied at three different maturation stages, which were Stage I (30–40 days), Stage II (40–50 days) and Stage III (60 days and above). Morphology, physicochemical properties, and bioactive compounds were evaluated to establish the postharvest quality. There was a significant difference in postharvest quality in terms of morphology and physicochemical properties at different maturation stages. Morphologically, the thallus of seaweed was straight and cylindrical. The secondary and tertiary branches were longer than the primary branches during stages II and III. During the whole development stage, G. changii was found in a pH range of 6.23–7.04, ash content (28.53–38.93%), (3.70–23.45 µg/g/100 ml) ascorbic acid, and (0.21–0.23 %) titratable acidity. Total phenolic content (TPC) was significantly highest at Stage III, whereas total flavonoid content (TFC) and the antioxidant activities measured by 2,2-diphenyl-1-picrylhydrazyl (DPPH) and 2,2'-Azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) assays remained unaffected by the maturation stage. This study revealed that different maturation stages affect the postharvest quality of seaweed. This information is valuable to guide the establishment of the harvesting standard for local seaweed production.