It is imperative to develop practicable pathogenic bacteria detection methods. We devised a biosensor for ultrasensitive detection of Salmonella typhimurium (S. typhi), termed as SCOUT-dCas9 (Ultra-Sensitive, Cross-validating, On-site and dUal-mode Test using CRISPR-dCas9). Simply, the species-specific invA gene of S. typhi was amplified using loop-mediated isothermal amplification (LAMP) with a biotinylated primer, which can be specifically “pulled down” by “antibody-like” dCas9-sgRNA to form a ternary complex. SYBR Green I (SG) and streptavidin-modified alkaline phosphatase (SA-ALP) were used to functionalize them to generate fluorescent and colorimetric signals, respectively. With this strategy, the target could be dexterously converted into bimodal signals that were cross-validated to afford more reliable results. For both modes, SCOUT-dCas9 was able to detect as low as 1 CFU/mL with a dynamic range from 1 to 109 CFU/mL. Lastly, SCOUT-dCas9 had satisfactory selectivity and was capable of detecting S. typhi-contaminated real food samples. SCOUT-dCas9 provides a robust platform for bacterial detection.