The Pacific archipelago Spain ceded to the United States under the 1898 Treaty of Paris, besides Guam, was the Philippines.
Signed on December 10, 1898, the treaty ended the Spanish-American War. Its Article III transferred the Philippine archipelago to the United States, while Spain separately ceded Guam and Puerto Rico and relinquished sovereignty over Cuba. The United States paid Spain $20 million for the Philippines.
A common mix-up involves the Mariana Islands: Guam is geographically part of that archipelago, but the treaty specifically named Guam as the island ceded to the United States. The Philippines were a separate, much larger archipelago in Southeast Asia. Hawaii was annexed by the United States in 1898 through a separate congressional resolution, and Samoa came under U.S. control later through the 1899 Tripartite Convention.
The treaty’s Philippine cession was controversial because Filipino revolutionaries had already declared independence and did not recognize the agreement. Fighting between Filipino and American forces began in February 1899.