Cambodia’s Supreme Court ruled to dissolve the main opposition party, effectively turning the country into the world’s newest one-party state and cementing long-serving leader Hun Sen’s grip on power.

Prime Minister Hun Sen’s government filed the suit to ban the Cambodia National Rescue Party after police arrested its chief figure, Kem Sokha, in Phnom Penh on Sept. 3 and charged him with treason. Prosecutors accused Mr. Kem Sokha of plotting to take over the country with the help of the U.S. But many political analysts and...