Outlining tumors in ultrasound B-mode images is an important process in many diagnostic procedures, but manual tumor segmentation is usually a time-consuming and challenging task. This paper presents a custom-made active contour model that is specifically designed for segmenting tumors in ultrasound images. The algorithm starts by drawing a circular contour around a manually selected point inside the tumor. The vertices of the circular contour are iteratively moved from the interior of the tumor to the tumor boundary. The motion of the vertices is controlled using an ultrasound-based statistical parameter, called the envelope signal-to-noise ratio (SNR), that is sensitive to variations in tissue anatomy. The proposed algorithm has been used to outline breast tumors in 10 ultrasound B-mode images. When compared to tumor outlines delineated by a human expert, the outlines obtained using the proposed algorithm achieved sensitivity values as high as 96.98%. The proposed algorithm provides a simple and accurate method for tumor segmentation in ultrasound images.