Eio or EIO may refer to:
- Eio (river) in Norway
- Eio Books, an American publishing house
- Eio Sakata (1920–2010), Japanese professional Go player
- Environmental Investment Organisation
- European Investigation Order
- Extended interaction oscillator
- Enhanced Input/Output, an extension interface for printers, see JetDirect § EIO
- Eiō, one of the eight major titles of professional shogi
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