The performance of an RT-LAMP assay does not require expensive special equipment such as a thermal cycler with real-time fluorescence measurement, because positive samples are determined by a color change from red to yellow within 30 min after the start of the incubation at 65°C. For detection, simple mobile phone cameras, copy machines, office scanners, or plate scanners with spectrophotometric quantification can be used. During the early phase of the COVID-19 pandemic (early March 2020) in Germany, we tested the sensitivity and specificity of a colorimetric RT-LAMP assay for detecting SARS-CoV-2 RNA in clinical RNA samples isolated from pharyngeal swab specimens collected from individuals being tested for COVID-19 (and provided by the Heidelberg University Hospital’s diagnostic laboratory after removal of an aliquot for SARS-CoV-2 RNA testing by RT-qPCR) (fig. S1). We also developed a swab–to–RT-LAMP assay that used naso/oropharyngeal swab specimens directly without the need for an RNA isolation step. We tested >700 clinical RNA samples with a wide range of viral loads, allowing us to determine accurately the sensitivity range of the colorimetric RT-LAMP assay. We also developed a multiplexed LAMP-sequencing protocol using barcoded Tn5 transposase tagmentation that enabled rapid identification of positive results in thousands of RT-LAMP reactions within the same next-generation sequencing run.