There is a dire need for simple and affordable diagnostic tests for disease management due to the inaccessibility of diagnostic tests in developing countries. Albeit the emergence of paper-based enzyme-linked immunosorbent assay (ELISA) sensors as an attractive alternative due to the assay specificity and simplicity of use, a paper-based sensor still suffers from certain limitations such as the requirements for sophisticated instruments or technologies in fabrication or detection. Here, we proposed a simple paper-based ELISA by modifying paper with chitosan and glutaraldehyde to enhance protein immobilization efficiency. Colorimetric results could be obtained in less than half an hour with a horseradish peroxidase (HRP) conjugated antibody. The assay was carried out with the use of a drinking straw as an incubation pouch. Quantification using Adobe Photoshop CS2 software was done after documentation using a mobile phone camera. The sensor was found to have a detection limit of 0.5 nM. This approach also holds promise for DNA sensing applications by hybridization.