Undifferentiated peripheral spondyloarthritis (SpA) and septic arthritis are two distinct differential diagnoses for patients with acute-onset monoarthritis. Effective history-taking and thorough physical examination are essential to differentiate between these two diseases. Precise follow-up can be critical for diagnosing undifferentiated peripheral SpA. Herein, we report our experience with two cases that required differentiation between undifferentiated peripheral SpA and septic arthritis. This case series shows the importance of ruling out septic arthritis promptly and considering the possibility of undifferentiated peripheral PsA based on clinical findings and imaging tests.