Siltstone is a sedimentary rock that is intermediate in grain size between the coarser sandstone and the finer shale. As its name implies, it is primarily composed of silt, defined as grains smaller than 62 micrometres. Siltstones were at one time often referred to as sandstones, but differ significantly due to their smaller pores and higher propensity for containing a significant shale fraction. They are important geological elements in certain stratigraphic series, such as in Muldraugh Hill in Kentucky. Siltstones may contain concretions.

Unless the siltstone is fairly shaly, stratification is likely to be obscure, and it tends to weather at oblique angles unrelated to bedding.