Lateral flow immunoassays (LFIAs) serve as rapid and convenient tools for disease screening. However, there is room for enhancing diagnostic sensitivity. Therefore, this paper introduces a novel material combined with lateral flow immunoassays to improve the sensitivity of COVID-19 detection and diagnosis. Trimethylsilyl cellulose (TMSC) was printed on the device between the test and control line to delay the flow of the solution, which resulted in a more excellent binding of the immunoassay. These delay-LFIAs (d-LFIAs) were developed for the diagnosis of COVID-19, targeting the detection of immunoglobulins G and M, as well as the COVID-19 antigen. Under optimized conditions, the d-LFIA increased the sensitivity by up to 5 times compared to that of conventional LFIAs. Moreover, it exhibited the ability to detect IgG, IgM, and SARS-CoV-2 antigens as low as 1 ng/mL (limit of detection) by the naked eye. In addition, the application of this device to real samples yielded highly consistent results with those obtained from other LFIAs and standard SARS-CoV-2 diagnostic methods (such as enzyme-linked immunosorbent assay and reverse transcription polymerase chain reaction) diagnosis. Therefore, we expect this device to be a new platform for highly sensitive, easy, and affordable immunoassay strip testing to certify future emerging diseases.