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Introduction: The study aims to demonstrate and estimate the prevalence of clinical corneal ectasia and keratoconus (KC) in patients with relatively low keratometry (low-K KC). Methods: In a retrospective, analytical, and non-interventionist study, one eye was randomly selected from 1054 patients from the original Tomographic Biomechanical Index (TBIv1) study and the external validation (from Rio de Janeiro, Brazil, and Milan, Italy clinics). Patients were stratified into three groups. Group 1 included 736 normal patients, and groups 2 and 3 included 318 patients with clinical KC in both eyes, divided into low-K KC (90 patients) and high-K KC (228 patients), respectively. All patients underwent a comprehensive ophthalmological evaluation along with Pentacam and Corvis ST (Oculus, Wetzlar, Germany) examinations. Cases with maximum mean zone 3 mm keratometry (Kmax zone mean 3 mm) lower than 47.6 diopters (D) were considered as low-keratometry keratoconus, and cases with Kmax zone mean 3 mm higher than 47.6 D were regarded as high-keratometry keratoconus. Results: Ninety (28.30%) of the 318 KC group presented ectasia with low-keratometric values (low-Kmax). The average age in the normal group was 39.28 years (range 6.99–90.12), in the low-Kmax KC group it was 37.49 (range 13.35–78.45), and in the high-Kmax KC group it was 34.22 years (range 12.7–80.34). Mean and SD values and median (range), respectively, of some corneal tomographic and biomechanical parameters evaluated from the low-Kmax KC group were as follows: Belin-Ambrósio enhanced ectasia display (BAD-D) 3.79 ± 1.62 and 3.66 (0.83–9.73); Pentacam random forest index (PRFI) 0.78 ± 0.25 and 0.91 (0.05–1); corneal biomechanical index (CBI) 0.58 ± 0.43 and 0.75 (0–1); TBI 0.93 ± 0.17 and 1 (0.35–1); and stiffness parameter at A1 (SP-A1) 86.16 ± 19.62 and 86.05 (42.94–141.66). Conclusion: Relatively low keratometry, with a Kmax lower than 47.6 D, can occur in up to 28.30% of clinical keratoconus. These cases have a less severe presentation of the disease. Future studies involving larger populations and prospective designs are necessary to confirm the prevalence of keratoconus with low keratometry and define prognostic factors in such cases. [ABSTRACT FROM AUTHOR]