This study reports the synthesis of a 1-inch one-unit-cell of non-layered chromium sulfide (Cr_2S_3) with unidirectional orientation on c-plane sapphire using an interface-modulated chemical vapor deposition (CVD) method. The interfacial interaction between Cr_2S_3 and the substrate induces intralayer sliding of self-intercalated Cr atoms, breaking the space reversal symmetry and leading to robust room-temperature ferroelectricity with ultrahigh remanent polarization. Additionally, long-range ferromagnetic order is observed with a Curie temperature approaching 200 K, almost twice that of the bulk material. The magnetoelectric coupling is also confirmed, making the 1-inch one-unit-cell of Cr_2S_3 the largest and thinnest multiferroic material. The work provides a novel strategy for the synthesis and investigation of wafer-scale one-unit-cell multiferroic materials, paving the way for their application in advanced devices such as spintronic chips and magnetoelectric devices.This study reports the synthesis of a 1-inch one-unit-cell of non-layered chromium sulfide (Cr_2S_3) with unidirectional orientation on c-plane sapphire using an interface-modulated chemical vapor deposition (CVD) method. The interfacial interaction between Cr_2S_3 and the substrate induces intralayer sliding of self-intercalated Cr atoms, breaking the space reversal symmetry and leading to robust room-temperature ferroelectricity with ultrahigh remanent polarization. Additionally, long-range ferromagnetic order is observed with a Curie temperature approaching 200 K, almost twice that of the bulk material. The magnetoelectric coupling is also confirmed, making the 1-inch one-unit-cell of Cr_2S_3 the largest and thinnest multiferroic material. The work provides a novel strategy for the synthesis and investigation of wafer-scale one-unit-cell multiferroic materials, paving the way for their application in advanced devices such as spintronic chips and magnetoelectric devices.