Three-dimensional (3D)-printed occlusal splints are becoming more prevalent in the treatment of tooth substance loss due to their fast and cost-effective production. The purpose of this in vitro study was to investigate whether the mechanical properties (tensile strength—TS, modulus of elasticity in tension—ME, and Vickers hardness—HV) vary between the materials (printed dimethacrylate-based resins: Keyprint KeySplint soft—KEY, Luxaprint Ortho Plus—LUX, V-Print splint—VPR, printed methacrylate-based resins Freeprint splint 2.0—FRE, and milled methacrylate-based material, CLEAR splint—CLE), and the influence of aging processes (extraoral storage conditions and nightly or daily use) was examined. The printed methacrylate-based resins (FRE, LUX, and VPR) had much higher TS (43.7–48.5 MPa compared to 12.3–13.3 MPa), higher ME (2.01–2.37 GPa compared to 0.43–0.72 GPa), and higher HV (11.8–15.0 HV compared to 3.3–3.5 HV) than both of the methacrylate-based resins (KEY and CLE) after the production process. Although the TS, ME, and HV of the printed dimethacrylate resins (FRE, LUX, and VPR) decreased significantly under humid conditions with possibly elevated temperatures (thermocycling as well as 37 °C), these mechanical properties were significantly higher than both methacrylate-based resins (KEY and CLE). Therefore, printed dimethacrylate resins should be used rather than methacrylate-based resins for high expected masticatory forces, low wall thicknesses, or very long wearing times (≥6 months).