External beam radiation therapy (RT) can induce toxicity in patients surgically treated for breast cancer. Modern irradiation techniques have lowered the incidence and severity of radiation-induced injuries; however, their side effects on normal tissues remain challenging. This review illustrates early and late changes observed using ultrasound (US) imaging, including echocardiography, at the skin, muscle, pleura, lungs, and heart levels. The US findings and the potential role of this technique in detecting and grading early and late complications of RT are highlighted in this article. US has proven useful in the differential diagnosis of post-RT complications, including but not limited to cancer recurrence and toxicity from other sources, such as anticancer drugs. Additionally, considering the progressive nature of RT-induced injury, early detection of toxicity may be helpful in the individual stratification of damage risk and serve as a tool for patient screening and management. In these cases, US can be used as a radiation-free biomarker of RT side effects at the subclinical stage.