Genome-wide identification of DNA double-strand breaks (DSBs) induced by CRISPR-associated protein (Cas) systems is vital for profiling the off-target events of Cas nucleases. However, current methods for off-target discovery are tedious and costly, restricting their widespread applications. Here we present an easy alternative method for CRISPR off-target detection by tracing the integrated oligonucleotide Tag using next-generation-sequencing (CRISPR-Tag-seq, or Tag-seq). Tag-seq enables rapid and convenient profiling of nuclease-induced DSBs by incorporating the optimized double-stranded oligodeoxynucleotide sequence (termed Tag), adapters, and PCR primers. Moreover, we employ a one-step procedure for library preparation in Tag-seq, which can be applied in the routine workflow of a molecular biology laboratory. We further show that Tag-seq successfully determines the cleavage specificity of SpCas9 variants and Cas12a/Cpf1 in a large-scale manner, and discover the integration sites of exogenous genes introduced by the Sleeping Beauty transposon. Our results demonstrate that Tag-seq is an efficient and scalable approach to genome-wide identification of Cas-nuclease-induced off-targets.