Mangrove-associated fungi are of great significance in industries due to their various and versatile enzyme production. So, this work studies the purification of β-mannanase produced by a halophilic Aspergillus terreus strain ARSA (accession No. MN075514) isolated from marine sediment of mangrove, Safaga, Red Sea coasts, Egypt. The enzyme was precipitated by 50% acetone, followed by purification using gel filtration on Sephadex G-100 yielding an active major protein peak showing 6.77-fold purification. The molecular weight of the purified β-mannanase was approximately 48 kDa, determined by a sodium dodecyl sulfate-polyacrylamide gel electrophoresis. K_m and V_max values were found to be 3.33 mg/ml and 1666.67 U/ml, respectively. The optimum pH and temperature of the purified enzyme were 6.5 and 60oC, respectively. The enzyme was stable from pH 5.0 to 7.5 and partially stable up to 80oC. The effect of activators and inhibitors was studied providing that EDTA, Hg2+, Tween 80, and SDS (10%) strongly inhibited the enzyme activity, while Mg2+, Mn2+, and Na+, enhanced enzyme activity. Mixtures of mannooligosaccharides are resulted from the hydrolysis of locust bean gum by the enzyme. Crude and purified A. terreus ARSA β-mannanase gave promising results on fruit juices clarification as on apple juice extraction with a yield of 128.32 and 167.65%, respectively.