OBJECTIVE: To determine whether an injectable thrombin product [thrombin hemostatic matrix (THM)] at closure of a Kocher-Langenbeck approach reduces the risk of heterotopic ossification (HO) formation after an acetabular fracture. DESIGN: Case control. SETTING: Two Level 1 trauma centers. PATIENTS: Patients with operatively treated acetabulum fractures fixed through Kocher-Langenbeck from 2013 to 2018. INTERVENTION: Records were reviewed for demographics, history of traumatic brain injury, HO medication or radiation prophylaxis, THM (Surgiflo, Ethicon, Bridgewater New Jersey) administration, and length of follow-up. Radiographs were reviewed for dislocation, fracture, Letournel and Orthopaedic Trauma Association classifications, HO, and Brooker grade if applicable. Patients receiving HO prophylaxis (eg, nonsteroidal anti-inflammatory drugs and radiation) were excluded. Remaining patients were divided into 2 groups: THM administration (intervention) and no THM. Continuous variables were compared using t-tests and categorical variables with chi-square or Fisher's exact tests. MAIN OUTCOME MEASUREMENTS: Risk ratios for the association between HO occurrence and THM administration. RESULTS: Three-hundred and twenty-eight acetabular fractures met inclusion criteria (126 intervention, 202 control) in patients with a mean age of 38.7 ± 15.9 years; 62.2% were male, and 42.1% were African American. Traumatic brain injury and posterior dislocation rates were equivalent between groups (P = 0.505, 0.754, respectively). HO rate in the control group was 42.6% compared with 21.4% in the THM group (P < 0.001). Booker grade 3/4 in control group was 17.3% versus 3.2% in the THM group (P < 0.001). Patients receiving THM had a 50% reduced risk of HO (95% confidence interval 0.35-0.73) compared to those who did not; adjustment for age, gender, ethnicity, and traumatic brain injury did not meaningfully change the association (risk ratio 0.46; 95% confidence interval 0.29-0.73; P < 0.001). CONCLUSION: The use of a surgiflo product at closure of a KO approach may reduce the risk of HO formation by 50% after an acetabular fracture. LEVEL OF EVIDENCE: Therapeutic Level III. See Instructions for Authors for a complete description of levels of evidence.