Background: Probiotics are purported to reduce symptoms of allergic rhinitis. This study sought to determine the proportion of participants with an improvement in the mini Rhinoconjunctivitis Quality of Life Questionnaire (mRQLQ) in response to a multispecies probiotic supplement with a Simon Two-Stage design. Methods: This study was based on a Simon Two-Stage Design for p1-p0 = 0.18 to account for seasonal variation in symptoms. Under this design, ≥10 patients are required to exhibit an improvement in quality-of-life scores to determine that there was sufficient activity for the supplement to be considered effective. Participants consumed a probiotic supplement (Ecologic® AllergyCare; probiotik®pur) twice daily for 8 weeks. The primary outcome measure was based on a change in mRQLQ scores following supplementation. Secondary outcomes include assessment of change in symptoms and medication usage with a twice-weekly symptom and medication diary, nasal congestion by rhinomanometry, and total serum Immunoglobulin E (IgE) and specific IgE for Bermuda grass. Results: A total of 40 participants completed the study. A total of 25 participants (63%, 49–76%, p < 0.001; mean, 95% confidence interval, p-value) out of 40 participants had a clinically meaningful response to treatment based on assessment of mRQLQ. On average, mRQLQ scores changed from 2.83 ± 1.51 at baseline to 1.66 ± 1.36 at week 4 and 1. 38 ± 1.13 at week 8 (p < 0.01) (mean ± SD, p-value). Sum of individual symptom scores and overall symptom scores over the course of treatment was significantly reduced (p = 0.036 and p = 0.039, respectively). A moderate reduction in frequency of allergy-related medication use in the final 4 weeks of supplementation period was observed (52.5% weeks 0–4 to 41.4% weeks 4–8; average proportion of total diary responses, p = 0.085). The supplement was largely well tolerated by participants at the dose provided. Conclusions: The proportion of participants exhibiting improvement in quality-of-life metrics warrants continued investigation in the form of a phase III placebo-controlled trial.