An Unreinforced masonry building (or UMB, URM building) is a type of building where load bearing walls, non-load bearing walls or other structures, such as chimneys are made of brick, cinderblock, tiles, adobe or other masonry material, that is not braced by reinforcing beams. The term is used as a classification of certain structures for earthquake safety purposes, and is subject to minor variation from place to place.

UMB structures are vulnerable to collapse in an earthquake. One problem is that most mortar used to hold bricks together is not strong enough. Additionally, masonry elements may "peel" from the building, and fall onto occupants or passersby outside.

In California, construction of new unreinforced masonry buildings were prohibited in 1933, and state law (enacted in 1986) required seismic retrofiting of existing structures. Retrofits are relatively expensive, and may include the building being tied to its foundation, tying building elements (such as roof and walls) to each other, so that the building moves as a single unit, rather than creating internal shears during an earthquake, attaching walls more securely to underlying supports, so that they do not buckle and collapse, bracing or removing parapets and other unsecured decorative elements. Retrofits are generally intended to prevent injury and death to people, but not to protect the building itself.

The California law left implementation and standards, up to local jurisdictions. Compliance took many years. As of 2008, most (but not all) of the unreinforced masonry buildings have undergone retrofitting.