"We underline the importance of peaceful dispute settlement as well as free and unimpeded lawful use of the world's oceans," they said in a communique.
China claims most of the South China Sea, while the Philippines, Brunei, Malaysia, Taiwan and Vietnam also have overlapping claims.
"We strongly oppose the use of intimidation, coercion or force, as well as any unilateral actions that seek to change the status quo, such as large-scale land reclamation," the G7 leaders said, without naming countries.
China has been criticized for extensive reclamation work and moves to turn submerged rocks into man-made structures.
The G7 Summit 2015 participants at Schloss Elmau, Germany.