Background Subcutaneous (SCIT) and sublingual (SLIT) immunotherapy are the 2 most prescribed routes for administering allergen-specific immunotherapy. They were shown to be effective in control of symptoms and in reducing rescue medication use in patients with allergic diseases, but their effectiveness has to be balanced against side effects. In recent years, SLIT has been increasingly prescribed, instead of SCIT, because of improved safety and easy administration. Objective We assessed which route is the most effective in the treatment of patients with seasonal allergic rhinitis to grass pollen. Methods An indirect meta-analysis–based comparison between SCIT and SLIT was performed. Treatment efficacy was determined as the standardized mean difference (SMD) in symptom and medication scores obtained with active treatment, SCIT or SLIT, compared with placebo. Studies were included if they were double-blind randomized controlled trials comparing SCIT or SLIT with placebo. Thirty-six randomized controlled trials (3014 patients; 2768 controls) were analyzed. Results The overall effect size of SCIT for symptom score (SMD, −0.92; 95%CI, −1.26 to −0.58) was significantly higher than SLIT, both administered via drops (SMD, −0.25; 95% CI, −0.45 to −0.05) and tablets (SMD, −0.40; 95%CI, −0.54 to −0.27). Similar results were reported for medication score (SCIT: SMD, −0.58; 95% CI, −0.86 to −0.30. SLIT drops: SMD, −0.37; 95% CI, −0.74 to −0.00. SLIT tablets SMD, −0.30; 95% CI, −0.44 to −0.16). Conclusions Our results provide indirect but solid evidence that SCIT is more effective than SLIT in controlling symptoms and in reducing the use of antiallergic medications in seasonal allergic rhinoconjuntivitis to grass pollen.