Clearly delineated forest boundaries are important for sustainable forest management. Interpreting aerial images for forest boundary delineation is a necessary, but labour-intensive process. The results are partly subjective and may include inconsistencies. In this article, we present an automatic approach to delineating forest boundaries in natural colour aerial images from the Swiss National Forest Inventory (NFI). This method is based on JSEG (J-measure-based Segmentation) image segmentation and can be used to obtain forest delineations using the green vegetation index (GVI) feature, Gabor wavelet texture features as well as curvature features from airborne laser scanning (ALS). This approach is compared with the commonly used method of supervised classification, nearest neighbour (NN), which requires local training, and is found to work well in several different landscape types after establishing thresholds. Although we tested the method in a variety of different landscape types, further development and testing are required to cope with complex terrain topography, for example, in regions at the upper tree line in the Alps.