We report on a neutron diffraction study of single-crystal CePtAl₃ complemented by measurements of the specific heat under applied magnetic field. Below T_N ≈ 3 K, CePtAl₃ develops incommensurate antiferromagnetic order with a single modulation vector k = (0.67600). Residual magnetic scattering intensity above T_N and a broadening of the specific heat anomaly at T_N may be consistently described in terms of a Gaussian distribution of transition temperatures with a standard deviation σ ≈ 0.5 K. The distribution of T_N may be attributed to the observation of occupational and positional disorder between the Pt and Al sites consistent with structural information inferred from neutron diffraction. Measurements under magnetic field reveal changes of the magnetic domain populations when the field is applied along the [010] direction consistent with a transition from cycloidal to amplitude-modulated magnetic order ∼2.5 T.