Most genotyping methods for known single‐nucleotide polymorphisms (SNPs) are based on hybridization with allele‐specific probes, oligonucleotide ligation reaction (OLR), primer extension or invasive cleavage. OLR offers superior specificity because it involves two recognition events; namely, the hybridization of an allele‐specific probe and a common probe to adjacent positions on target DNA. OLR products can be detected by microtiter well‐based colorimetric, time‐resolved fluorimetric or chemiluminometric assays, electrophoresis, microarrays, microspheres, and homogeneous fluorimetric or colorimetric assays. We have developed a simple, robust, and low‐cost disposable biosensor in dry‐reagent format, which allows visual genotyping with no need for instrumentation. The OLR mixture contains a biotinylated common probe and an allele‐specific probe with a (dA)₂₀ segment at the 3′‐end. OLR products are denatured and applied to the biosensor next to gold nanoparticles that are decorated with oligo(dT) strands. The sensor is immersed in the appropriate buffer and all components migrate by capillary action. The OLR product is captured by immobilized streptavidin at the test zone (TZ) of the sensor and hybridizes with the oligo(dT) strands of the nanoparticles. A characteristic red line is generated due to the accumulation of nanoparticles. The excess nanoparticles are captured by immobilized oligo(dA) at the control zone of the strip, giving a second red line. We have applied successfully the proposed OLR‐dipstick assay to the genotyping of four SNPs in the drug‐metabolizing enzyme genes CYP2D6 (^*3 and ^*4) and CYP2C19 (^*2 and ^*3). The results were in agreement with direct sequencing. Hum Mutat 0,1–8, 2008.