Background/Objectives: β-Caryophyllene is a plant-derived sesquiterpene with recognized therapeutic potential; however, high lipophilicity and low aqueous solubility limit its parenteral application. Nanoemulsion-based systems represent a rational strategy to address these challenges. This study aimed to develop and physicochemically characterize an injectable β-caryophyllene nanoemulsion and to evaluate its preclinical safety following intramuscular administration. Methods: Five nanoemulsions (NBCP1–NBCP5) were prepared by low-energy emulsification using different hydrophilic–lipophilic balance (HLB) values and characterized in terms of particle size, polydispersity index, zeta potential, and encapsulation efficiency. The optimized formulation (NBCP1) was evaluated in a 14-day subacute intramuscular toxicity study in male Wistar rats (n = 5 per group) at doses of 5 and 15 mg/kg. Clinical observations, food and water intake, body weight, hematological and biochemical parameters, and histopathological analyses of muscle, liver, and kidney tissues were assessed. Results: NBCP1 exhibited favorable physicochemical properties, including a mean particle size of 102.39 nm, PDI of 0.27, zeta potential of −27.5 mV, and encapsulation efficiency of 97.16%, and remained stable under stress conditions. Repeated intramuscular administration did not induce behavioral alterations, changes in consumption patterns, or differences in body weight between the control and treated groups. Hematological and biochemical parameters remained within physiological ranges, and histopathological analysis revealed preserved tissue architecture without inflammatory or degenerative changes. Conclusions: The results support the suitability of NBCP1 as a stable nanoemulsion platform for the parenteral delivery of β-caryophyllene under the evaluated conditions. These findings address the limited information available on injectable formulations of this sesquiterpene and provide a foundation for future pharmacokinetic, longer-term safety, and efficacy studies.