The total content of acidic compounds in sake is called titratable acidity, which is a parameter to evaluate the taste of sake. Thus, the titratable acidity of sake and moromi (sake mash) is frequently determined for their control during brewing. In this study, a portable sensor has been developed for determining the titratable acidity of sake and moromi based on the voltammetric measurements of the reduction prepeak of 3,5-di-tert-butyl-1,2-benzoquinone (DBBQ) in the presence of acidic compounds. The portable sensor, which consists of a prototype potentiostat, a function generator, a mobile lithium-ion battery, a recorder, and an electrochemical cell, was used to measure a voltammogram of DBBQ. The prepeak currents of DBBQ were proportional to the concentrations of succinic acid ranging from 1.33 to 33.3 mM, and thus the portable sensor could be applied to determine titratable acidity ranging from 0.266 to 6.66. In real sample analyses using commercially available sakes and moromi samples fractionated during brewing, the titratable acidities determined using the portable sensor were essentially the same compared with those determined by neutralization titration. These results demonstrate that our portable sensor is useful for on-site analysis for the quality control of sake.