Mn-N-alloyed transformation-induced plasticity (TRIP)-assisted duplex stainless steel is a promising Ni-saving advanced high-strength steel for impact-resistant structural applications due to its high strength, good plasticity, and the TRIP effect. However, under high-strain-rate compression, adiabatic temperature rise can enhance austenite stability, suppress martensitic transformation, and render conventional dynamic constitutive descriptions insufficient. This work investigates a Mn-N-alloyed TRIP-assisted duplex stainless steel through quasi-static compression and split Hopkinson pressure bar tests across a wide strain-rate range, from 2.0 × 10⁻³ s⁻¹ to 4.0 × 10³ s⁻¹. Specimens were compressed along the rolling direction. The strain-rate-dependent mechanical response was analyzed alongside the evolution of martensite fraction, phase transformation behavior, local misorientation, crystallographic texture, and deformation substructures. SEM, EBSD, TEM, magnetic measurements, and thermodynamic calculations of the stacking fault energy were combined to reveal the underlying deformation mechanisms. Based on these findings, a modified Johnson–Cook constitutive model incorporating adiabatic heating was established to describe the steel’s dynamic flow behavior. Results show that the duplex stainless steel exhibits an excellent combination of high strength and plasticity, with no shear fracture observed during high-strain-rate impact compression. Both dynamic yield strength and compressive strength increase with increasing strain rate. At a strain rate of 4.0 × 10³ s⁻¹, the yield strength and compressive strength along the rolling direction reach 800 and 1143 MPa, respectively. The deformation mechanism changes substantially with strain rate. Under quasi-static compression, significant martensitic transformation occurs via both γ→ε→α¢ and γ→α¢ transformation paths, and the volume fraction of α′ martensite reaches 8.66%. By contrast, at 4.0 × 10³ s⁻¹, adiabatic heating enhances austenite stability and suppresses the TRIP effect, reducing the α′ martensite fraction to 0.92%. The transformation path is then dominated by the direct γ→α¢ transformation, whereas the γ→ε→α¢ route is strongly inhibited. Thermodynamic calculations show that the stacking fault energy increases from 20.92 mJ/m² at room temperature to 41.08 mJ/m² under the highest-strain-rate condition. This shifts the deformation mode from TRIP-dominated deformation toward a synergistic mechanism involving deformation twinning, the residual TRIP effect, and dislocation slip. TEM observations further confirm that plastic deformation at high strain rates is primarily facilitated by the formation of dislocation walls in ferrite, along with dislocation slip, stacking faults, and deformation twins in austenite. The modified Johnson–Cook model, which introduces adiabatic temperature rise into the thermal softening term, accurately reproduces the nonlinear dynamic flow stress response while avoiding the overestimation associated with the conventional isothermal Johnson–Cook model. These findings provide a mechanistic basis and a constitutive description for the application of TRIP-assisted duplex stainless steels in impact-resistant structural design.