French administrator and diplomat who, in the formative years of colonialism in French Indochina, played a crucial role in mitigating relations between the European colonialists and the French administration, on the one hand, and the indigenous population and its royal court at Hue, in central Vietnam. He was considered generally sympathetic to the Vietnamese.

Philastre graduated from France's naval school in 1857 and signed onto the Avalanche, bound for China. He arrived in Cochinchina (now in southern Vietnam) in 1861 and was named inspector of indigenous affairs in January 1863 at My Tho, a Mekong River delta village. Two years after being made chief of native law (June 1868), he grew ill and returned to France. During the Franco-German War and the Paris Commune, Philastre commanded an artillery regiment in the defense of Paris. He returned to Saigon in 1873, working under Admiral Jules-Marie Dupré. (Dupré, Marie-Jules)

An avid imperialist, Dupré had been seeking official recognition from the Vietnamese emperor Tu Duc of French holdings in three western provinces formally ceded to France in an 1862 treaty that Tu Duc had reneged upon. Meanwhile, in the summer of 1873, the profiteering trader Jean Dupuis (Dupuis, Jean), eager to secure northern Vietnam for trade with China, attacked Hanoi. Dupré then sent an expedition north under Francis Garnier (Garnier, Francis), ostensibly to suppress Dupuis's bold move. Garnier, however, with Dupré's approval, joined forces with the trader and laid claim to Hanoi. But Dupré had meanwhile received word from France officially condemning the whole affair, and he commissioned Philastre as ambassador to try to win concessions from Tu Duc.

Philastre was poorly received at Hue by Tu Duc, who asked him to go north to resolve matters with the French forces there. At Hanoi, feeling that the Vietnamese had been wronged, he acted against Dupré's implicit directives and, presenting himself as Dupré's representative, ordered a cease-fire. Arriving shortly after Garnier had been killed in battle, Philastre was able to assert his authority. He laid the foundations of the treaty of 1874, under which Tu Duc at last bowed to the French conquest of the south. So impressed were the Vietnamese by Philastre's honesty and sincerity that they agreed to terms in the treaty that exceeded what the French had asked. Thereafter he served as representative of the protectorate of Cambodia (1876) and as chargé d'affaires at Hue (1877–79). He returned to France in 1880 and taught mathematics at Cannes and Nice (1882–94).

Briefly discredited because of his counter-coup in the Dupuis-Garnier affair, which delayed French expansion into the north of Indochina for a decade, Philastre won the acclaim of later French writers for his honesty and for the respect he showed for the people and traditions of Vietnam. His most important scholarly achievement was the translation into French of the Vietnamese legal code and its commentaries. The erudite work was published as Le Code annamite in two volumes in Paris in 1876.