Background The purpose of this study was to evaluate the influence of posterior medial corner (PMC) injuries on clinical outcome and second-look arthroscopic findings after anterior cruciate ligament (ACL) reconstruction. Methods Seventy-eight consecutive patients underwent a second-look arthroscopic surgery after ACL reconstruction and magnetic resonance imaging (MRI) examination of the PMC. The patients were divided into a PMC intact group ( n = 42) and a PMC injured group ( n = 36). The stability and clinical outcomes were evaluated using the Lachman test, pivot-shift test, a KT-2000 arthrometer, and the Lysholm knee scoring scale. Graft tension and synovial coverage were evaluated in second-look arthroscopy. Results The clinical function showed no significant differences regarding PMC injury. Although the graft tendon tension revealed no significant differences ( p = 0.141), the second-look arthroscopic findings indicated that the PMC intact group showed better synovial coverage compared to the PMC injured group ( p = 0.012). Conclusion Patients who injured the PMC had poor synovial coverage as assessed by second-look arthroscopic findings after transtibial ACL reconstruction, even though clinical outcomes and stability showed no significant differences.