This paper describes a study that focused on developing, validating and standardizing a dyscalculia test, henceforth called the Dyscalculia Test. Out of the 4,758,800 students in Nigeria's upper primary and junior secondary schools, I randomly drew a sample of 2340 students, using a multistage sampling procedure that applied various sampling techniques. For data collection, I used the Test of General Reasoning Ability and Paper 1 of the Mathematics Achievement Test section of the 2021 National Common Entrance Examination, as well as the Dyscalculia Test introduced in this paper, which was developed and standardized in stages. My analysis shows that the Dyscalculia Test items effectively zero in on three components: number sense, arithmetic operation, and working memory. Based on my findings, I recommend that school administrators and counselors adopt the Dyscalculia Test to assess students who may be having difficulty in mathematics and arithmetic for proper diagnosis.