This paper introduces a new proactive solid-state circuit breaker (P-SSCB) for direct current (dc) medium voltage applications. The P-SSCB aims to apply high-power rating thyristors while achieving a fast response speed and high efficiency. To manually turn off thyristors, a complimentary commutation technique is used; meanwhile, full-controlled switches are employed in the auxiliary branch to minimize the reaction time interval. To implement the complementary commutation and achieve a reliable turn-off process in the main thyristor, the injection capacitor in the auxiliary branch is charged by the dc system. The charging process of the injection capacitor is controlled by a thyristor, which helps to obtain a fast and reliable re-closing and re-breaking. During fault current interruption, the injection capacitor is prevented from being reversely charged using an anti-parallel diode to facilitate using polar capacitors. The proposed P-SSCB is validated in a medium-voltage dc test setup by experiments of 1.5 kV/265 A. The results show that the peak voltage across the main and auxiliary branches reaches 2895 V and 2852 V, respectively.