Compartment syndrome is a well-described clinical condition and is considered an orthopedic emergency affecting individuals of all ages. A typical scenario for acute compartment syndrome involves lower limb fractures or crush injuries. However, physicians may occasionally encounter atypical presentations, defined as atypical compartment syndrome (ACS). A 38-year-old, left-handed male patient without any comorbidities developed ACS of the forearm and clinical presentation of sepsis after a small penetrating injury to his right forearm. He developed ACS secondary to infected hematoma and subsequent soft tissue infection caused by Proteus mirabilis and Morganella morganii. Both bacteria infected the patient by direct contamination after injury with a knife, resulting in multifloral contamination. The patient was successfully treated with reconstructive surgery. In conclusion, ACS secondary to this type of penetrating injury shows a subtle clinical course at the time of hospital admission and can insidiously progress from an infected hematoma, posing a serious threat to the limb or even cause mortality. Good extremity function without any disability can be achieved with an accurate diagnosis during the initial evaluation of the patient in the emergency department and prompt surgical intervention followed by appropriate reconstructive methods.