Hall-Petch analysis and the effect of the average grain size (D) on the hardness (H) of AISI 309Si stainless steel should be systematically investigated. This aim was achieved by cold rolling (reduction of thickness of 90%) followed by annealing at 1000 °C for different holding times. Cold rolling led to the formation of deformation-induced α΄-martensite and work-hardening of the retained austenite, which led to an increased hardness of 427 HV compared to the hardness of 115 HV for the as-received sample. X-ray diffraction (XRD) analysis revealed the formation of ~30 vol% martensite, which signifies the higher mechanical stability of austenite in this alloy when compared to the metastable grades. At the annealing temperature of 1000 °C, the material is fully austenitic without any intermetallics. Moreover, by occurrence of martensite reversion, recrystallization of the retained austenite, and subsequent grain growth, it is easy to obtain different grain sizes at this temperature. Annealing at 1000 °C for 30 min resulted in a significant increment of hardness (155.1 HV) compared to the as-received sample as well as the formation of a fine-grained microstructure with an average grain size of 6.7 µm, which is equivalent to ~93% refinement of the grain size. Longer annealing times led to the occurrence of grain growth with the resulting decrement of hardness, where D–D0=2.1762×1011exp(–280000/RT)×t0.5 was proposed to predict the grain size during grain coarsening. Moreover, the Hall–Petch relationship of H=140.1/√D+100.7 was proposed for the first time, in which the H0 value of 100.7 HV was also compared to those determined for AISI 316L (130.0 HV) and 304L (160.4 HV) alloys. Furthermore, it was revealed that the increased metastability leads to an increment in the H0 value, which can be correlated with the Md30/50 temperature.