Objective: This study aimed to determine the stability of microspheres of Sargassum plagyophyllum (brown seaweed) after preparation using spray drying with maltodextrin DE 10-15 and during drying and storage.Methods: Aqueous extracts of brown seaweed were formulated into microspheres using maltodextrin DE 10-15 as a coating agent. For increasing the stability of polyphenol compounds, spray drying was performed with an inlet temperature of 110°C. Four microsphere formulations were produced using maltodextrin DE 10-15 at concentrations of 0%, 5%, 10%, and 15%. The resulting microspheres were then characterized in the assessments of moisture contents, particle size distributions, pH, total phlorotannin contents, and antioxidant activity, and surface morphology was analyzed using scanning electron microscope analyses. Results:Powders that were produced with 0% and 15% maltodextrin were more stable at 4°C±2°C than at 28°C±2°C and 40°C±2°C. At the lowest temperature, phlorotannin contents were maintained in powders that were prepared with 15% maltodextrin but were decreased by 10% in powders that were prepared without maltodextrin. Conclusion:Maltodextrin DE 10-15 is a suitable coating agent for dry formulations of S. plagyophyllum powder and maintained stability during spray drying at 110°C and during storage for 2 months at 4°C±2°C. InstrumentsBrookfield viscometer (Brookfield, USA), sputter coating instrument (Quorum Q150R ES), scanning electron microscope (SEM) (Carl Zeiss EVO MA 10), Mastersizer 2000 (Malvern Instruments Ltd., Worcestershire, UK), pH meter (Eutech Instruments pH 510, Singapore), and moisture tester (AMB 50) wer used. Seaweed materialsThe brown seaweed S. plagyophyllum was collected from Pantai Binuangeun, Lebak, Banten, Indonesia. The species was identification by an algae researcher from Oseanografi Research Laboratory Center, LIPI Jakarta. The collected seaweed was thoroughly washed in water to remove salts, epiphytes, and sand and was then air-dried in the shade for 3-4 days [10]. Preparation of seaweed extractsA dried seaweed (600 g) was extracted using a maceration method for 24 h with stirring in 6 L of demineralized water at room temperature in the dark [10,11]. Macerates were then filtered to produce the aqueous extracts of S. plagyophyllum, which were then characterized for pH, total phlorotannin contents, and antioxidant activity. Determinations of total phlorotannin contentsTotal phlorotannin contents of S. plagyophyllum extracts were determined using the Folin-Ciocalteu method with phloroglucinol as a standard [11]. Briefly, aqueous extracts (5 g) were dissolved in 10 mL of demineralized water with shaking, and the resulting mixtures