Leptospirosis is a common life-threatening disease worldwide. However, its diagnosis is frequently ineffective because the gold standard bacterial culture and microscopic agglutination test (MAT) are usually positive 1-2 weeks after the disease onset. We thus developed an immunochromatographic assay (LEPkit) to detect serum anti-leptospiral lipopolysaccharide (LPS) IgM for rapid diagnosis of acute leptospirosis. Using referenced sera of 77 leptospirosis and 91 non-leptospirosis cases, LEPkit yielded 97.4% sensitivity, 94.5% specificity, 93.8 positive predictive value (PPV), 97.7% negative predictive value (NPV), and 95.8% accuracy. The stability of this kit stored for up to 18 months and its reproducibility were confirmed. Testing in 74 new cases using samples at admission-phase and subsequent paired samples (total n = 135), overall sensitivity was 98.5%, whereas that of culture and single MAT (≥1:400) was 15.6% and 35.6%, respectively. When only the samples at admission-phase were used (n = 74), the sensitivity remained at 98.7%, whereas that of culture and single MAT (≥1:400) was 28.4% and 13.5%, respectively. In summary, our LEPkit was far more effective than any conventional methods for the diagnosis of acute leptospirosis, especially within the first few days after the disease onset. The ease of use, stability and reproducibility further enhance its feasibility for clinical use on-site.