A 3‐year‐old, crossbreed dog was presented for an acute onset of cervical pain 6 weeks after the initiation of corticosteroid treatment for an immune‐mediated polyarthritis. Except for cervical hyperesthesia, neurological examination was unremarkable. Computed tomography revealed thrombosis of cervical vertebral venous structures and caudal cerebral sinuses. The dural sac containing the cervical spinal cord was moderately to severely compressed. A decrease in antithrombin activity was measured and assumed to be caused by secondary altered production due to corticosteroid therapy as well as important active thrombosis. The hypercoagulable state was most likely caused by chronic corticosteroid administration as a treatment of immune‐mediated polyarthritis. Complete resolution of clinical signs and venous lesions was achieved by tapering off corticosteroids and initiating gabapentin and antithrombotic treatment (clopidogrel and rivaroxaban).