The paper deals with a military career of Dalmatius the Junior, a nephew of Emperor Constantine I the Great. In 335, Constantine confered a title of Caesar on Dalmatius and gave him the Lower Danube regions - Thracia, Minor Scythia, Lower Moesia, Маcedonia, Dacia and Greece - for supervision. But two years later, in 337, Dalmatius the Junior was murdered as a result of a state plot held in Constantinople. Having based on narrative sources (an anonymous Epitoma de Caesaribus, the so-called Anonymus Valesianus, part I, and the works of Zosimus and Johannes Lydus), which contain very scarce evidence about Dalmatius the Junior's military and political achievements, the author proposes an idea that the nephew of Constantine I held an office of master of soldiers (magister militum), a supreme commander of expeditionary (field) army comitatenses, which stood in Thracia. Therefore, the author suggests that Dalmatius the Junior was one of the two first masters of soldiers appointed by Constantine I. Presumably, Dalmatius the Junior received the office in 335 simultaneously with a title of Caesar. Having based on the newest epigraphical document, that is an inscription from Lower Moesia published just recently, in 2016, the author concludes that the basis of Dalmatius the Junior's troops were detachments of the mobile Dalmatian cavalry.