De Martiis is an Italian surname belonging to the wide onomastic group formed by De Martis, De Marzi, De Marziis, De Martini, De Martinis, De Martino and Demartis. All these surnames derive from the old Latin names Martius or Marcius, related to Mars (Mars), Roman god of war, from whom also come historical names such as that of Ancus Marcius, fourth legendary king of Rome.

The particle De has patronymic value and expresses belonging or descent. The form De Martiis preserves a Latinizing spelling in which Martiis corresponds to the Latin genitive plural, a grammatical case used to indicate possession or belonging. The surname originally meant "of the Martii" or "of the family of the Martii", equivalent to the Italian dei Martii.

According to Rossoni, De Martiis presents a nucleus in Piedmont and another in Rome. The author considers that some variants, such as De Martis and De Marziis, could have originated through graphic simplifications or transcription errors of the older form De Martiis. The related group De Marzi is found mainly in the province of Sondrio, in the Vicenza area, in Venice and, above all, in Rome and in the localities of Marino and Velletri.

The wide diffusion of the different variants reflects the notable vitality in Italy of the surnames derived from Latin personal names linked to Martius and Marcius, whose origins go back to the onomastic tradition of ancient Rome.