<p>Layered dielectric materials and their van der Waals (vdW) heterostructures offer high potential for next-generation two-dimensional (2D) electronic devices, but materials that combine a wide bandgap and high dielectric constant are rare. Here, we present the controllable synthesis of quasi-vdW layered samarium oxysulfate (Sm₂O₂SO₄) single crystals via a molten-salt-assisted chemical vapor deposition (CVD) method. These atomically thin crystals exhibit remarkable dielectric properties, including a wide bandgap (~5.54 eV), high dielectric constant (~18), robust breakdown voltage (&gt;12 MV cm^-1) and good thermal reliability. By integrating ultrathin Sm₂O₂SO₄ nanoplates with 2D molybdenum disulfide (MoS₂) via vdW forces, we fabricate field-effect transistors (FETs) showing a subthreshold swing down to 65.2 mV dec^-1, hysteresis down to 5.4 mV, on/off current ratios of ~10⁹, and gate leakage currents down to around 7 × 10^-7A cm^-2. Furthermore, a high gate coupling ratio (GCR ~ 0.83) non-volatile memory device was developed based on the MoS₂/h-BN/MLG/Sm₂O₂SO₄/MLG heterostructure. The flash memory achieves ultrafast (~50 ns) programming/erasing operations, robust endurance (&gt;2000 cycles) and long-term retention (&gt;10 years). This work shows promising results for the integration of Sm₂O₂SO₄ as a high-<i>κ</i> dielectric in future 2D devices, with implications for low-power, high-performance electronics.</p>