Context. S TE P AR S YN is an automatic code written in Python 3.X designed to infer the stellar atmospheric parameters T eff , log  g , and Fe/H of FGKM-type stars following the spectral synthesis method. Aims. We present a description of the S TE P AR S YN code and test its performance against a sample of late-type stars that were observed with the HERMES spectrograph mounted at the 1.2-m Mercator Telescope. This sample contains 35 late-type targets with well-known stellar parameters determined independently from spectroscopy. The code is available to the astronomical community in a GitHub repository. Methods. S TE P AR S YN uses a Markov chain Monte Carlo sampler to explore the parameter space by comparing synthetic model spectra generated on the fly to the observations. The synthetic spectra are generated with an spectral emulator. Results. We computed T eff , log  g , and Fe/H for our sample stars and discussed the performance of the code. We calculated an internal scatter for these targets of −12 ± 117 K in T eff , 0.04 ± 0.14 dex in log  g , and 0.05 ± 0.09 dex in Fe/H. In addition, we find that the log  g values obtained with S TE P AR S YN are consistent with the trigonometric surface gravities to the 0.1 dex level. Finally, S TE P AR S YN can compute stellar parameters that are accurate down to 50 K, 0.1 dex, and 0.05 dex for T eff , log  g , and Fe/H for stars with v sin i ≤ 30 km s −1 .