Corneal mechanical strength is critical to withstanding intraocular pressure and maintaining normal shape. In keratoconus, the mechanical stability is compromised, which may lead to progressive morphological changes. Therefore, a noninvasive technique capable of accurately measuring the mechanical properties of the cornea may help us understand the mechanism of keratoconus development and improve detection and intervention in keratoconus. We previously developed Brillouin microscopy based on light scattering from inherent acoustic waves in tissues and showed that this technique can provide quantitative estimates of local longitudinal modulus, which correlate to the Young and/or shear moduli of the cornea. Using a clinically viable instrument, for the first time, to our knowledge, we mapped the elastic modulus of normal and keratoconic corneas in vivo. We found distinctive biomechanical features that differentiate normal and keratoconic corneas and therefore have the potential to serve as diagnostic metrics for keratoconus.