<jats:p><jats:italic>Conyza blinii</jats:italic> H.Lév. is a widely used medicinal herb in southwestern China. The main pharmacological components of <jats:italic>C. blinii</jats:italic> are a class of oleanane‐type pentacyclic triterpene glycosides known as conyzasaponins, which are thought to be synthesized from β‐amyrin. However, no genes involved in the conyzasaponin pathway have previously been identified. Here, we identify an oxidosqualene cyclase (OSC), a β‐amyrin synthase, which mediates cyclization of 2,3‐oxidosqualene to yield β‐amyrin. Ten OSC sequences were isolated from <jats:italic>C. blinii</jats:italic> transcript tags. Phylogenetic analysis was used to select the tag Cb18076 as the putative β‐amyrin synthase, named <jats:italic>CbβAS</jats:italic>. The open reading frame of <jats:italic>CbβAS</jats:italic> is 2286 bp and encodes 761 amino acids. Its mature protein contains the highly conserved motifs (QXXXGXW/DCTAE) of OSCs and (MWCYCR) of β‐amyrin synthases. Transcription of <jats:italic>CbβAS</jats:italic> was upregulated 4–24 h after treatment of the seedlings of the <jats:italic>C. blinii</jats:italic> cultivar with methyl jasmonate. Furthermore, expression of <jats:italic>CbβAS</jats:italic> in <jats:italic>Saccharomyces cerevisiae</jats:italic> successfully yielded β‐amyrin. The chemical structures and concentrations of β‐amyrin were confirmed by GC‐MS/MS. The target yeast ultimately produced 4.432 mg·L<jats:sup>−1</jats:sup> β‐amyrin. Thus, CbβAS is an OSC involved in conyzasaponin biosynthesis.