Presidential spokesman Edwin Lacierda said President Benigno Aquino III signed a law Wednesday granting citizenship to the American-born Blatche.
Senator Juan Edgardo Angara, the main author of the law, says Filipinos are a "sports-loving nation with a distinct affection for basketball." He says the 6-foot-11 Blatche, a power forward and center, can make "significant contributions to Philippines basketball."
The International Basketball Federation, or FIBA, allows each team to have one naturalized player per tournament. The Philippines is competing at the World Cup for the first time in 35 years.
Blatche averaged 11.2 points and 5.3 rebounds per game this season for Brooklyn.
Arriving in the Philippines on Sunday to meet the team, Blatche said that playing for the country "wasn't a tough decision" to make.
"The sport of basketball is huge here," he told reporters at a news conference Tuesday. "It's in the heart of every Filipino and I love basketball, and so it's like we have that similarity."
The Philippines are in Group B at the World Cup, along with Argentina, Croatia, Greece, Puerto Rico and Senegal. The tournament will be held in Spain in late August through early September.
After the World Cup, Blatche will again play for the Philippines team at the Asian Games in South Korea in September.
The Associated Press contributed to this report.