An extra 16 gendarmes and 44 border police will be sent to Mayotte, starting to arrive in August, Valls told local political leaders during his visit, on which he was accompanied by Education Minister Najat Vallaud-Belkacem and Overseas Territories Minister George Pau-Langevin.
The prime minister was on a three-day tour of France's Indian Ocean territories and had already visited the island of Réunion.
Two "plagues" affect the daily lives of local people, Valls said, a "worrying level of crime" and "illegal immigration", which threatens the "cohesion" of the main island, Maore, also known as Grande-Terre.
An estimated 80,000 undocumented immigrants are on the island, which has 210,000 official inhabitants.
Boats carrying immigrants continue to leave the nearby Comoros island of Anjouan, he said, despite the French state using radar, boats and personnel to try to stop them.
The 44 extra border police will work at a new detention centre to be opened in September, replacing a centre that was too small and whose conditions Valls declared "unworthy of the republic".
Valls also promised to continue to invest in education in Mayotte, where 87,000 students currently attend schools, which often lack adequate facilities for children from poor backgrounds.