A decoction of Rhizophora mucronata Lam. mangrove bark is used as an antidiabetic treatment in Asia. Kombucha tea is a fermented beverage, which is also claimed to be antidiabetic. In this work, the potency of R. mucronata leaves as α-glucosidase inhibitor was studied to assess whether it could be a suitable alternative to the use of R. mucronata bark. α-glucosidase inhibitory activities were determined for three extracts prepared from R. mucronata leaves, being the unfermented tea of R. mucronata leaves, the fermented kombucha tea and an 80% methanolic extract of the residual R. mucronata leaves. Flavonoid glycosides were identified in tea powder, kombucha tea and in the crude methanolic extract. Both the unfermented tea and the kombucha tea after 7 days of fermentation inhibited α-glucosidase with IC50 values of 0.12 ± 0.02 mg/mL and 0.09 ± 0.04 mg/mL, respectively. The methanolic extract showed a stronger α-glucosidase inhibitory activity compared to the kombucha tea and tea powder with an IC50 value of 0.0435 ± 0.0007 mg/mL. Acarbose, used as a positive control, inhibited α-glucosidase with an IC50 value of 2.4 ± 0.2 mg/mL. It was found that the three types of preparations of R. mucronata all were potent α-glucosidase inhibitors.