Otto Sillier (7 November 1857 – 4 March 1925) was a German trade union leader.
Born in Berlin, Sillier completed an apprenticeship as a printmaker, and joined the Senefelder Union.
He visited the United States in 1909 in an attempt to recruit new affiliates to the federation; he was unsuccessful, but did obtain agreements to reduce competition.
The stress of this led Sillier to suffer a mental breakdown in 1912, and although he gradually recovered, his physical health worsened during World War I.
Sillier remained active in the international, trying to retain affiliates from countries on both sides, by adopting a neutral tone in its journal.