| Extension | Summary |
|---|
| .Y | Yacc Source File |
| .Y3A | Yu-Gi-Oh! ONLINE 3 Avatar File |
| .Y3D | Yu-Gi-Oh! ONLINE 3 Deck File |
| .Y4M | YUV4MPEG2 Video File |
| .YAB | Yabasic Source Code |
| .YAL | Arts & Letters Clipart Library |
| .YAM | Yahoo! Auctions Data |
| .YAML | YAML Document |
| .YAODL | PowerFlip 3D Image File |
| .YBD | YobiDrive Link File |
| .YBK | Microsoft Encarta Yearbook File |
| .YCHAT | Yahoo! Messenger Chat Log |
| .YCM | Yugioh Card Maker File |
| .YDC | Yu-Gi-Oh! Card Deck File |
| .YDK | Yu-Gi-Oh! Deck File |
| .YDL | PowerFlip YAODL 3D Image File |
| .YDR | Grand Theft Auto V 3D Model |
| .YDT | Yu-Gi-Oh! Power of Chaos Data File |
| .YENC | yEnc File |
| .YFS | YSFLIGHT Mission File |
| .YGF | Y Graph Format File |
| .YKA | Yenka Model File |
| .YKCOL | Locky Ransomware Encrypted File |
| .YM | Atari ST Audio File |
| .YMG | Yahoo! Messenger File |
| .YML | YAML Document |
| .YML2 | YML Script |
| .YMP | YaST Metapackage File |
| .YNC | yEnc Encoded File |
| .YOG | Outerra Captured Video File |
| .YOOKOO | Yookoo Player Playlist File |
| .YOP | Psygnosis YOP Video |
| .YPR | BYOB Project File |
| .YPS | Yahoo! Messenger Data File |
| .YRCBCK | YRC Backup Archive |
| .YRCBKM | YRC Bookmarks Archive |
| .YRCDAT | YRC Software Data Script |
| .YSP | BYOB Sprite File |
| .YTD | Grand Theft Auto 5 Texture File |
| .YTF | Google Picasa Font Cache |
| .YTT | YouTube Timed Text File |
| .YUMTX | YUM Transaction File |
| .YUV | YUV Encoded Image File |
| .YZ | YAC Compressed File |
| .YZ1 | Yamazaki ZIPPER File |