The magnetic penetration depth (λ) as a function of applied magnetic field and temperature in SrPt3P (Tc ≃ 8.4 K) was studied by means of muon-spin rotation (μSR). The dependence of λ−2 on temperature suggests the existence of a single s-wave energy gap with the zero-temperature value "0 = 1.55(2) meV. At the same time λ was found to be strongly field dependent which is the characteristic feature of the nodal gap and/or multiband systems. The multiband nature of the superconducting state is further suggested by the upward curvature of the upper critical field. This apparent contradiction is resolved by SrPt3P being a particular two-band superconductor with equal gaps but different coherence lengths within the two Fermi surface sheets.