High purity magnesium hydroxide has been synthesised from seawater with the addition of dolomite lime as reagent. High intensity ultrasound-assisted crystallization of magnesium hydroxide was carried out during precipitation with the intention to obtain fine particles of Mg(OH)2 as well as to prevent their agglomeration. Investigations were conducted on magnesium hydroxide samples in the form of sludge and voluminous powder. The composition, morphology, and properties of the product were determined by chemical, XRD, FTIR, and SEM/EDS analysis. The particle size distribution was detected by the laser light scattering method. The results showed that the mean particle size of magnesium hydroxide sludge was 5.75 µm, while voluminous powder was 7.58 µm. The morphology of magnesium hydroxide voluminous powder was in the form of aggregated filamentous (up to 300 nm in size) or flake structures.; Magnezijev hidroksid visoke čistoće sintetiziran je iz morske vode uz dodatak dolomitnog vapna. Ultrazvukom visokog intenziteta potpomognuta kristalizacija magnezijeva hidroksida tijekom precipitacije odvijala se u svrhu dobivanja što finijih čestica Mg(OH)2 i sprječavanja njihove aglomeracije. Ispitivanja su provedena na uzorcima magnezijeva hidroksida u obliku mulja i voluminoznog praha. Sastav, morfologija i svojstva dobivenog produkta analizirani su kemijskom, XRD, FTIR i SEM/EDS analizom. Raspodjela veličine čestica određena je metodom raspršenja laserskog svjetla. Rezultati ukazuju na to da je srednja veličina čestica magnezijeva hidroksida u obliku mulja 5,75 µm, dok je u voluminoznom prahu 7,58 µm. Morfologija voluminoznog praha magnezijeva hidroksida je u obliku aglomeriranih nitastih (duljine do 300 nm) i ljuspičastih struktura.