Deodato Arellano (July 26, 1844-October 7, 1899) was a Filipino patriot who was among the founders of the Katipunan that started the Philippine Revolution against Spain.
He was born in Bulacan, Bulacan to Juan and Mamerta de la Cruz, who changed their family name to Arellano when the Spanish colonial government decreed in 1849 that Filipinos should change their names with ones picked from a Madrid directory. Arellano studied bookkeeping at the Ateneo Municipal de Manila (now Ateneo de Manila University) and later worked as assistant clerk at the arsenal of the colonial army in Manila. He was married to Hilaria del Pilar, a sister of Marcelo H. del Pilar.
Along with del Pilar, Arellano was an active freemason. He also solicited funds for Filipino expatriates in Spain when del Pilar had to flee to Spain because of allegedly subversive articles that appeared in the newspaper that del Pilar published, Diariong Tagalog (Tagalog Newspaper), in the Philippines.
In 1892, Arellano and other freemasons, like Andres Bonifacio, joined La Liga Filipina, which was founded by Jose Rizal upon his return to the country. Arellano was elected La Liga secretary but Rizal was deported to Dapitan in Mindanao a few days later.
On July 7, 1892, when the government announced Rizal's deportation, Arellano, Bonifacio, Ladislao Diwa, Teodoro Plata, Valentin Diaz and Jose Dizon founded the Katipunan. He was elected the first president of its supreme council.