Background/Objectives: Orally administered herbal preparations have gained attention as complementary approaches in supportive cancer care; however, their clinical efficacy and safety remain uncertain. This systematic review critically evaluated evidence from randomized clinical trials investigating these interventions in patients with cancer. Methods: The review followed Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 2020 guidelines and was prospectively registered in PROSPERO. PubMed/MEDLINE, ScienceDirect, Springer Nature Link, CENTRAL, Scopus, LILACS, ClinicalTrials.gov, and WHO ICTRP were searched for English-language studies published from January 1990 through 8 September 2026. Risk of bias was assessed using RoB 2, and outcome-level certainty was evaluated using GRADE. Results: Eight randomized clinical trials met the eligibility criteria and were included in the qualitative synthesis. The studies evaluated Gamiguibi-tang, Rikkunshito, holy basil (Ocimum tenuiflorum), Alcea digitata–Malva sylvestris, Annona muricata, Malva sylvestris–Althaea digitata, Jing Si herbal tea, and Renshen Yangrong Tang across heterogeneous cancer populations. Individual trials reported potentially favorable findings for sleep, fatigue, chemotherapy-induced anorexia, anxiety, salivary function, xerostomia, gastrointestinal symptoms, and quality of life. However, the Jing Si herbal tea trial found no statistically significant between-group differences in fatigue or quality of life, and exploratory ex vivo cytotoxicity findings did not demonstrate clinical antitumor efficacy. Meta-analysis was not feasible because of substantial clinical and methodological heterogeneity. All evaluated outcomes were rated as very-low-certainty evidence, primarily because of risk of bias and serious imprecision. No serious intervention-related adverse events were identified in the published reports, but adverse-event monitoring was inconsistent and no trial was adequately powered to establish safety. Conclusions: The available evidence is insufficient to establish definitive efficacy or safety or to support routine clinical implementation. Adequately powered multicenter trials using botanically authenticated and chemically characterized preparations, clinically meaningful outcomes, interaction assessment, and long-term pharmacovigilance are required.