测深学（英语：Bathymetry；来自希腊语：βαθύς，bathus，「深度」和希腊语：μέτρον，metron，「测量」）是研究水面下海床或湖床深度的学科，测深学相当于海面上的测高学或测绘学。水深或水文图的制作目的通常是为了水面和水下的航行安全，并且会以等值线（即等深线）和探测得知的特定深度值表示海床地势；而且同时还提供海面的航海信息。不以航行安全为考量制作的水深图可能同时使用数字地面模型和人工照明技术来表示深度。古测深学（Paleobathymetry）则是测量古代海洋深度的学科。

Bathymetry is the study of underwater depth of lake or ocean floors. In other words, bathymetry is the underwater equivalent to hypsometry or topography. The name comes from Greek βαθύς (bathus), "deep", and μέτρον (metron), "measure". Bathymetric (or hydrographic) charts are typically produced to support safety of surface or sub-surface navigation, and usually show seafloor relief or terrain as contour lines (called depth contours or isobaths) and selected depths (soundings), and typically also provide surface navigational information. Bathymetric maps (a more general term where navigational safety is not a concern) may also use a Digital Terrain Model and artificial illumination techniques to illustrate the depths being portrayed. The global bathymetry is sometimes combined with topography data to yield a Global Relief Model. Paleobathymetry is the study of past underwater depths.