A study was conducted to investigate fungal diversity in mangrove (Avicennia marina) and the potential of fungal isolates obtained from mangrove in inhibiting Pythium aphanidermatum and damping-off disease of cucumber. MiSeq analysis of fungal diversity in mangrove roots, stems and leaves showed that the three parts have a relatively high level of fungal diversity. Ascomycota was the most dominant phylum in all the parts, with Aspergillus, Fusarium and Cladosporium being the most dominant fungal genera. Detection of fungi in mangrove roots, stems and leaves using culture-based techniques showed that Aspergillus and Fusarium were the most dominant isolates on mangrove roots. Six, one and no fungal genera were detected in mangrove roots, stems and leaves, respectively. MiSeq analysis detected more fungal genera in mangrove compared to culture-based techniques, which could be related to the ability of MiSeq analysis to detect slow growing as well as obligate fungal species.Testing the antagonistic activity of ten fungal isolates from mangrove against Pythium aphanidermatum showed that one isolate exhibited in vitro inhibition of P. aphanidermatum growth. Electron microscope examination revealed that the antagonistic fungal isolate resulted in shrinking and groves in Pythium hypha. The fungal isolate was identified as Aspergillus terreus. When A. terreus culture filtrate was added to P. aphanidermatum, it resulted in a significant increase (by 73%) in electrolyte leakage from Pythium hypha compared to the control, as well as significant reduction (by 71%) in oospore production. The A. terreus culture was also found to produce cellulase enzyme, which is suggested to be involved in the antagonism against P. aphanidermatum. Adding A. terreus to soil infested with P. aphanidermatum significantly reduced percent mortality in cucumber seedlings by 70%. A. terreus, when applied alone on cucumber seedlings, did not show any suppressive effects on cucumber growth (length and fresh and dry weight). This appears to be the first report of isolation of A. terreus from mangrove with antagonistic activity against P. aphanidermatum-induced damping-off of cucumber. The study indicates that fungal isolates obtained from marine environments may serve as potential biocontrol agents against some plant pathogens.