Focal myositis is a rare disease defined by an isolated inflammatory pseudotumour usually restricted to one skeletal muscle. Approximately, 250 cases of focal myositis have been described in the literature, and two recent large cohorts have been used to help in the diagnosis. Isolated gastrocnemius myositis, a rare immune-mediated condition, is a diagnostic entity used by internal medicine clinician in the gastrocnemius myalgia syndrome associated with Crohn's disease (CD). However, focal myositis and isolated gastrocnemius myositis with Crohn's disease share clinical, haematological, pathological, and radiological similarities. We present a case of unilateral focal myositis of the gastrocnemius muscle in a patient with no underlying diseases, including Crohn's disease. At clinical evaluation, we encountered a challenge in differentiating between focal myositis and the isolated gastrocnemius myositis of Crohn's due to similarities in clinical manifestation. We attempt to clarify focal myositis and isolated gastrocnemius myositis through our case report and a review of literature.