<p>The popularity of 316L steel in the production of parts by selective laser melting is attributed to its unique combination of properties. This paper focuses on a comparative analysis of austenitic 316L steel manufactured by two routes: conventional method in accordance with ASTM A276/A276M-17 Condition A involving rolling and annealing at 1050°C with water cooling and selective laser melting in the starting condition (as-printed). The samples were heat-treated employing well-established and experimental parameters to relieve residual stresses (900°C, water cooling) and to stabilize geometric dimensions (427°C, air cooling). The steel microstructure was examined with an AxioMat 200M optical microscope and a TESCAN electron microscope. Kalling’s reagent was applied for different etching times (10, 30, and 60 sec) to reveal various structural elements and inheritance. Phase diagrams were calculated with the JMatPro software package with thermodynamic databases. The microhardness of structural components was determined with a Zwick/Roell EMCO-TEST microhardness tester under a load of 50 g (with a mapped area of 2.4 mm × 2.4 mm). Simulation of the continuous cooling transformation diagram for 316L steel, taking into account the actual chemical composition and metallographic analysis data, demonstrated that all test samples consisted of austenite after manufacturing and subsequent heat treatment, regardless of the cooling rate and production route. For the samples produced conventionally, the austenite grain size was 40 ± 10 μm in the as-delivered condition, 43.2 ± 8 μm after heating to 427°C, and 56.2 ± 4 μm after heating to 900°C. In the as-delivered condition, the samples exhibited pronounced grain size heterogeneity in the diametral section. The heterogeneity persisted following heating to 427°C but significantly decreased following heating to 900°C. The microhardness of the samples ranged from 208 to 248 HV0.5 in the as-delivered condition, from 250 to 270 HV0.5 after heating to 427°C, and from 180 to 190 HV0.5 after heating to 900°C. Microhardness distribution maps indicated that the microhardness remained heterogeneous after heating to 427°C, although its values increased. The samples heated to 900 °C showed a more uniform distribution of microhardness, but the absolute values decreased. The 316L steel samples produced by selective laser melting preserved the original arc-like structure after heating to 427°C and 900°C. However, after heating to 900°C, the arc-like bands became thinner, and the contrast between the boundaries and the core of the track (formed by a single laser pass over the powder layer) decreased. The microhardness of the samples produced by selective laser melting was 239–251 HV0.5 (in the longitudinal section) and 286–317 HV0.5 (in the transverse section) and changed to 218–248 HV0.5 (in the longitudinal section) and 213–254 HV0.5 (in the transverse section) after heating to 427°C and to 183–208 HV0.5 (in the longitudinal section) and 187–220 HV0.5 (in the transverse section) after heating to 900°C. Following heating to 900°C, the microhardness distribution became uniform in both directions, which may be indicative of structural recrystallization with preserved inheritance (increased microhardness was observed at layer intersections).</p>