Topsoil is the uppermost layer of soil, usually the top six to eight inches. It has the highest concentration of organic matter and microorganisms, and is where most of the Earth's biological soil activity occurs. Plants generally concentrate their roots in, and obtain most of their nutrients from this layer. Topsoil can be measured as the depth from the surface to the first densely packed soil layer known as hardpan.

A major environmental concern known as topsoil erosion , occurs when the topsoil layer is blown or washed away. Without topsoil, little plant life is possible.

A variety of soil mixtures are sold commercially as topsoil, usually for use in improving gardens and lawns.

As one of the layers of soil on the Earth's surface, topsoil is sometimes referred to as the A horizon.