This study compares austenite grain growth and its continuous cooling transformation (CCT) behavior between selective laser melting (SLM) 4340 and conventional wrought 4340 steels. Standard dilatometry tests were used to determine the austenite decomposition behavior at different cooling rates. The analysis of the data from the controlled cooling paths was used to generate CCT diagrams for the two steels investigated in this study. Advanced microstructural characterization techniques, Scanning Electron Microscopy and Orientation Imaging Microscopy, were employed to support the CCT diagrams. Electron Backscatter Diffraction (EBSD) and a special EBSD-Image Quality characterization technique were used to assess the percentage of the microstructural components observed. Three important observations were made: (1) the hardenability of the wrought 4340 steel was higher than the SLM 4340 steel; (2) the presence of granular bainite was observed after slow cooling conditions in both steels; (3) the SLM steel exhibited finer austenite grain size distribution than the wrought steel in the temperature range studied. MTEX, a MatLab program, was used to reconstruct the prior austenite grains. The SLM steel exhibited finer austenite grain size distribution than the wrought steel in the temperature range studied.