The use of medicinal plants dates back to the beginning of humanity, and today their application as complementary therapy has been widely disseminated as an alternative to conventional therapy. The medicinal plant named Uncaria tomentosa (Willd.) DC. (known as cat's claw) is a common woody vine of the Amazon forest that has traditionally been used in the treatment of arthritis because of its anti-inflammatory properties. This study aimed to evaluate the cytotoxic, mutagenic, and antimutagenic potentials of this medicinal plant. The biological activities of U. tomentosa were determined on bone marrow cells of Wistar rats that were treated in vivo. For the cytotoxic and mutagenic analyses, aqueous plant extract solutions were administered by gavage (1, 2, or 3 mg/mL) for 24 h (an acute treatment) or 7 days (a subchronic treatment). For the antimutagenic analyses, aqueous plant extract solutions (1 mg/mL) were administered by gavage before (pretreatment), simultaneous to (simultaneous treatment), or after (post-treatment), the administration of cyclophosphamide (1.5 mg/mL). U. tomentosa did not show any cytotoxic or mutagenic effects in any of the cytological or chromosomal analyses. Besides, the antimutagenic tests showed that the plant extracts displayed antimutagenic activities, which significantly reduced the percentages of the chromosomal aberrations that were induced by cyclophosphamide at 53.91, 58.60, and 57.03%, respectively, for the simultaneous treatment, pretreatment, and post-treatment. The results suggested a safe use of this herbal medicine that is available free of charge from the Brazilian Public Health System for the treatment of arthritis. This medicinal plant can also effectively contribute to improving the quality of life and the recovery of people undergoing chemotherapeutical treatments.