The legislation allows Japan to use force by exercising its right to collective self-defence, public broadcaster NHK reported.
Japan will be able to defend other nations with which it has close relations, if its survival is threatened.
The Japanese Self-Defence Forces can now carry out new duties.
Self-Defence Forces on UN peacekeeping missions will also be able to use their weapons to come to the aid of foreign forces under attack from any armed group. This duty may apply to Japanese peace keepers in South Sudan.
Government officials are expected to draw up new rules of engagement for these duties over the next three months. Drills based on the new rules are expected to begin later this year.