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Background: Artemisia annua is an important autumnal pollen allergen for seasonal allergic rhinitis (SAR) in northern China. To date, no study has investigated allergen immunotherapy with A annua. We aimed to investigate the efficacy and mechanisms underlying A annua‐sublingual immunotherapy (SLIT). Methods: This was a randomized, double‐blind, placebo‐controlled phase III clinical trial involving 71 SAR patients, randomized to SLIT with A annua extract (n = 47) or placebo (n = 24) for 32 weeks. Total nasal symptom score (TNSS; primary clinical end point) was evaluated at baseline (peak pollen phase (PPP) in the previous year), initiation of A annua‐SLIT, 1st PPP during SLIT, end of SLIT and 2nd PPP during follow‐up. Blood samples and nasal secretions were collected at beginning and after SLIT for assessment of T cells and inflammatory mediators. Safety was assessed according to adverse events (AEs) reported. Results: Artemisia annua‐SLIT significantly reduced TNSS to a greater level from baseline (from 9.45 ± 1.68 to 6.16 ± 2.27) than placebo (from 9.29 ± 2.09 to 9.05 ± 2.40) at the 1st PPP (P <.001) and sustained the improvement in symptoms throughout to the 2nd PPP. Preseasonal A annua‐SLIT for 16 weeks significantly decreased Th2 cells, increased nTreg and Tr1 cells in blood; and increased cystatin 1 (CST1) in nasal secretion after 16 and 32 weeks compared with pretreatment. Overall, 17/47 patients experienced mild local AEs and 2 patients mild systemic AEs, after A annua‐SLIT. Conclusion: Artemisia annua‐SLIT is an efficacious and safe treatment in patients with A annua SAR. [ABSTRACT FROM AUTHOR]