Abstract Microorganisms are now the main sources of industrial enzymes, with fungi and yeasts that have contributed about 50%, bacteria has contributed about 35%, whereas the remaining 15% are also of phytopathogenic fungi while some are plant parasitic nematodes or animal origin. The prospective of using these microorganisms as sources of biotechnological and industrial relevant enzymes motivated interest in the study of extracellular enzymes from numerous microorganisms which include various fungi, bacteria, actinomycetes and yeasts. The aim of this study was to isolate and screen for mannanase-producing yeasts from citrus wastes and to produce, optimize, purify and characterize the enzyme. One hundred and two (102) yeasts were isolated from different citrus wastes obtained from five different markets and screened for their mannanase producing ability. Gum Arabic was used as a substrate for the production of mannanase enzyme by yeasts isolates using submerged fermentation. Candida sp. LES2, Candida sp. OS12, Pichia sp. and Pichia kudriavzevii (AUMC 10190) were the yeast isolates with the best potential of mannanase production. Incubation period, pH, temperature, carbon and nitrogen source were optimized under submerged fermentation for the production of mannanase. The mannanase produced by Pichia kudriavzevii (AUMC 10190) was optimally active at pH 7.0 (202.10 U/ml) and its temperature is stable at 600C. Pichia kudriavzevii (AUMC 10190) was optimally active at 350C (147.91U/ml). Mannanase enzyme produced by Pichia kudriavzevii (AUMC 10190) had a purification fold of 2.14 with a specific activity of 134.53U/ml proving that yeasts obtained from citrus wastes have the ability to produce mannanase enzyme under optimized conditions. Therefore, it can be concluded that Pichia kudriavzevii (AUMC 10190) isolated from citrus waste is potential yeast for the production of mannanase under suitable condition for higher yield.