Abstract The soft gamma-ray repeater Swift J1555.2−5402 was discovered by means of a short burst detected with Swift BAT on 2021 June 3. Then, 1.6 hr after the burst, the Neutron star Interior Composition Explorer (NICER) started daily monitoring of this target for a month. The absorbed 2–10 keV flux stayed nearly constant at around 4 × 10⁻¹¹ erg s⁻¹ cm⁻² during the monitoring, showing only a slight gradual decline. An absorbed blackbody with a temperature of 1.1 keV approximates the soft X-ray spectrum. A 3.86 s periodicity is detected, and the period derivative is measured to be 3.05(7) × 10⁻¹¹ s s⁻¹. The soft X-ray pulse shows a single sinusoidal shape with an rms pulsed fraction that increases as a function of energy from 15% at 1.5 keV to 39% at 7 keV. The equatorial surface magnetic field, characteristic age, and spin-down luminosity are derived under the dipole field approximation to be 3.5 × 10¹⁴ G, 2.0 kyr, and 2.1 × 10³⁴ erg s⁻¹, respectively. We detect 5 and 45 bursts with Swift/BAT and NICER, respectively. Based on these properties, this new source is classified as a magnetar. A hard X-ray power-law component that extends up to at least 40 keV is detected with the Nuclear Spectroscopic Telescope Array (NuSTAR). The 10–60 keV flux is ∼9 × 10⁻¹² erg s⁻¹ cm⁻² with a photon index of ∼1.2. The pulsed fraction has a sharp cutoff at around 10 keV with an upper limit (≲10%) in the hard-tail band. No radio pulsations are detected during the DSN or VERA observations. The 7σ upper limits of the flux density are 0.043 and 0.026 mJy at the S and X bands, respectively.