Sometimes, the raster data you have for a theme is just much larger than the actual extent of your study area or map. Or, in the case of scanned maps, you have extra nonmap information around the outside edge. In these cases, you want to cut out a portion of your raster.
You'll need a raster file that you want to cut a portion of. In this example, we will use the North Carolina whole state elevation model (elev_state_500m.tif) and cut it with the outline of Wake County (county_wake.shp). Load both of these files in a fresh QGIS project.
The easiest way to do this is to use a polygon mask layer. The vector mask can be a rectangle, but it doesn't have to be. The outline of a single polygon works best, though.
Tip
An alternate method would be to determine the bounding box (bbox) coordinates of the extent that you want with the Capture Coordinate tool or to draw the rectangle directly on the map.
Go to Raster | Extraction | Clipper.
Set Input file (raster...