<p>This study presents a numerical analysis at the planar interface of chiral-black phosphorus (BP) and investigates the characteristics of surface plasmon polaritons (SPPs) in the terahertz (THz) spectral region. The extended electromagnetic (EM) wave theory is employed for the numerical calculations, while the impedance boundary condition approach is used to derive the dispersion relation. The results demonstrate that the strong anisotropic dispersion of BP gives direction-dependent plasmon modes. The normalized propagation constant (NPC) of plasmon modes is found to be highly sensitive to electron doping, number of BP layers, chirality, and the refractive index of the chiral medium for the conductivity components of BP along the zigzag <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\({(\sigma }_{zz})\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mrow> <mo stretchy="false">(</mo> <mi>σ</mi> </mrow> <mrow> <mi mathvariant="italic">zz</mi> </mrow> </msub> <mrow> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> and armchair <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(({\sigma}_{ac})\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo stretchy="false">(</mo> <msub> <mi>σ</mi> <mrow> <mi mathvariant="italic">ac</mi> </mrow> </msub> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> directions. Moreover, the electromagnetic field propagation characteristics can be modified effectively by tuning both the chiral and BP parameters. The presented results can be used to fabricate tunable THz devices, providing an extra degree of freedom arising from the anisotropic conductivity of BP.</p>