Calotropis procera, known as “Silk cotton”, stands out for the presence of various classes of bioactive compounds responsible for its ethnopharmacological properties. The study aimed to conduct a phytochemical investigation, evaluating the in vitro and in vivo toxicity together with the antinociceptive potential of an n-butanolic fraction (FB) from the leaves. The crude ethanolic extract (CEE) was obtained by maceration in ethanol for 72 h. It was then partitioned using a gradual solvent sequence. The FB was analyzed by HPLC-ESI-MS/MS in negative mode and ¹H and ¹³C NMR. Toxicity was assessed by the erythrocyte hemolytic assay and acute oral toxicity test at a single dose of 300 mg·kg⁻¹. The antinociceptive effect was assessed by the acetic acid-induced abdominal writhing test and the formalin test in mice at doses of 3.75, 7.5 and 15 mg·kg⁻¹ per os. HPLC-ESI-MS/MS analysis identified flavonoids, phenolic acids, and the megastigmane roseoside, isolated for the first time in C. procera. The FB did not cause hemolytic effects or behavioral or physiological changes in mice. It showed an antinociceptive effect at all doses, reducing abdominal writhing by up to 91.46% and the licking time in phases 1 and 2 of the formalin test by up to 63.83% and 91.73%, respectively. In this study, it was possible to determine that an FB of a crude extract of C. procera leaves has antinociceptive activity, possibly associated with the phenolic compounds and roseoside found, with a lack of toxicity in vitro and in vivo, validating its ethnopharmacological use.