In most developing countries Leptospirosis disease is under-reported because available techniques are laborious, time-consuming and difficult to be applied under the precarious laboratory diagnostic facilities. The agglutination test using temperature resistant antigen (ATR) is a simple test used for leptospirosis screening in Argentina, but it lacks efficiency being mandatory its replacement with new rapid and accurate tests. Lateral flow immunoassays (LFIAs) are appropriate tools for diagnosis decentralization because they are easy one-step techniques that do not require equipment, complex facilities or expertise. In the present work, we developed a LFIA for leptospirosis diagnosis in Argentina. The test was designed for detecting specific human IgM in serum samples from acute patients with suspected leptospirosis. The diagnostic yield of the obtained test (Lepto-LF) was evaluated at different stages of the disease and compared with ATR. Lepto-LF resulted to be highly specific and overperformed ATR at all the evaluated stages, importantly during the early infection. At the early acute phase including samples with 0-5 days post onset of symptoms (dpo), Lepto-LF resulted with sensitivity (Se)= 45.5%, specificity (Sp)= 100%, Youden’s index (J)= 0.45, a positive likelihood ratio(LR+) tending to infinite and negative likelihood ratio (LR-)= 0.6, while ATR showed to be inadequate for leptospirosis diagnosis (Se= 36.4%, Sp= 27.3%, J= -0.36, LR+= 0.5, LR-= 2.3).At the late acute phase (6-10 dpo) Lepto-LF achieved perfect diagnostic yield (Se= 100%, Sp= 100%, J= 1, LR+ tending to infinite and LR- tending to 0) in comparison with ATR which barely gave moderate accuracy (Se= 94.7%, Sp= 57.9%, J= 0.53, LR+= 2.3, LR-= 0.1). Lepto-LF is a rapid test, simpler, more accurate and objective compared with ATR, being a suitable tool for replace it. Lepto-LF would help to overcome the technical and diagnostic limitations of ATR, such as poor reproducibility, low specificity and lack of diagnostic accuracy, improving the current screening of leptospirosis in Argentina. Our test may be also applied in other developing countries with similar epidemiological features. Moreover, its perfect specificity from the beginning of the disease suggest that Lepto-LF could be applied as confirmatory test yielding earlier and timely results for patient care, particularly in low-income areas and rural zones with limited or no access to confirmatory diagnosis by MAT.