The ultrasound and computed tomography findings of a retroperitoneal pseudoaneurysm associated with a grass awn are described in a 10-month-old dog. Ultrasound was used to localize the lesion and surrounding reaction as well as to determine its relationship with the celiac artery, but inadequate Doppler settings hindered the diagnosis of its vascular nature. Dual phase CT enabled further characterization, particularly its close relationship with the major retroperitoneal vessels. The imaging examination was fundamental in recommending nonsurgical therapy. The dog died as a consequence of the rupture of this pseudoaneurysm. A grass awn was confirmed.