Helicobacter pylori (HPY) is a type of spiral bacterium. The International Agency for Research on Cancer (IARC) has classified HPY as a Group 1 carcinogen, indicating that it is the leading cause of stomach cancer. The traditional testing method for HPY is cumbersome and requires professional personnel to operate. Owing to advances in biochemical technology, rapid tests for HPY have been developed. The development of the rapid test is based on colloidal gold. The rapid test displays two lines when the result is positive. The color of the test result can vary due to changes in the size or concentration of colloidal particles. Users may misinterpret the results owing to the effect of ambient lighting or when the color is too close to the background color. In this study, we successfully developed a method for using a smartphone to evaluate the results and sample concentration in rapid tests for detecting HPY. The HPY rapid test is placed on a colorimetric board developed for this study, and an image is captured using a smartphone and evaluated using the image processing program designed in this research. The ArUco marker is used for positioning and color correction to ensure that it is not affected by the brand of the camera of the mobile phone and nonuniform ambient light sources. To confirm whether the proposed method correctly determines the result and sample concentration, experiments were conducted using multiple different mobile phones. In the experiment, the illumination and shooting height were fixed and several different sample concentrations were used. Printed simulated and real rapid test results were used in the experiment. The experimental results show that both positive and negative can be correctly determined, and there is good linearity when determining each concentration of samples from the image. For example, when using simulated rapid tests, the R2 value is at least 0.9915, and the detection concentration threshold of the real HPY rapid tests is 5000 CFU/mL. Even with the concentration at 1250 CFU/mL, the R2 value is still 0.9473.