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Advancements in radiation therapy allow for targeting smaller tumor volumes with exceedingly conformal precision. Target motion during the breathing cycle needs to be anticipated. To date, no information exists that characterizes the motion of canine thoracic tumors during the respiratory cycle. The primary purpose of this study is to assess the pattern of motion of thoracic tumors in three-dimensional (3D) space between inspiratory and expiratory breath holds as a surrogate measurement of the breathing cycle. A secondary aim is to evaluate whether tumor position within the thorax relates to the degree of motion. Fourteen dogs were prospectively enrolled and received thoracic computed tomography (CT) scans with pre- and post-contrast inspiratory and expiratory phases achieved via manual breath holds. Gross tumor volume (GTV) and normal structures were delineated on inspiratory and expiratory image sets, and shifts were measured and analyzed. Smaller tumors had more motion in the dorsoventral direction compared to larger tumors (p = 0.047; rs = -0.44). The same observation was appreciated in the craniocaudal direction (p = 0.008; rs = -0.56). Dogs with multiple tumors had more motion in the dorsoventral direction and the craniocaudal direction compared to dogs with solitary tumors (p = 0.035; p = 0.022). Caudal tumors had significantly more 3D motion (p < 0.001) and craniocaudal motion (p < 0.001) compared to cranial tumors, whereas there was significantly more lateral motion (p < 0.001) between peripheral and central tumors, but no significant difference in 3D motion (p = 0.24). The variability of tumor motion between canine patients supports individualized assessments of each case to ensure best practice.