| .y | Amiga Yabba Compressed Archive Format |
| .y01 | Corel Paradox Secondary Index |
| .y02 | Corel Paradox Secondary Index |
| .y03 | Corel Paradox Secondary Index |
| .y04 | Corel Paradox Secondary Index |
| .y05 | Corel Paradox Secondary Index |
| .y06 | Corel Paradox Secondary Index |
| .y07 | Corel Paradox Secondary Index |
| .y08 | Corel Paradox Secondary Index |
| .y09 | Corel Paradox Secondary Index |
| .y3a | Yu-Gi-Oh! Online Avatar Format |
| .y3d | Yu-Gi-Oh! Online Card Format |
| .y3p | Yu-Gi-Oh! Online Profile |
| .y3t | Yu-Gi-Oh! Online Tag |
| .y4m | YUV4MPEG2 Video Format |
| .y8pd | Password Finder Yahoo! Mail / Yahoo! Messenger Exported Password |
| .yab | Yabasic Source Code |
| .yadpcm | Raw Yamaha 4-bit ADPCM Format |
| .yah | HeartBeat |
| .yajl | YAJL Script |
| .yak | KayakFoundry 3D Design |
| .yal | Arts & Letters Clipart Library Format |
| .yam | Yahoo! Auctions Data Format |
| .yaml | YAML Document |
| .yaodl | Powerflip 3D Image Format |
| .yar | StarBoard Document |
| .yast | YaST |
| .yaws | Yaws Source |
| .ybc | SUSE Linux Enterprise Server Display Settings |
| .ybd | YobiDrive Link Format |
| .ybhtm | YBook HTML |
| .ybk | Microsoft Encarta Yearbook Format |
| .ybm | Ad-Aware 2007 |
| .ybrar | Ybook Ebook |
| .ybrtf | Ybook Ebook |
| .ybtxt | Ybook Ebook |
| .ychat | Yahoo! Instant Messenger Chat Log Format |
| .ycm | Yugioh Card Maker Format |
| .ycp | SUSE Linux Enterprise Server Display Settings |
| .yct | YahELite Chat Transcript |
| .ydc | Konami Yu-Gi-Oh! Card Deck Format |
| .ydl | Powerflip YAODL 3D Image Format |
| .ydr | Yu-Gi-Oh! Power Of Chaos Replay Format |
| .ydt | Konami Yu-Gi-Oh! Power Of Chaos Data Format |
| .yel | Yellow Color Separation |
| .yenc | YEnc Encoded Archive |
| .yfs | YSFLIGHT Mission Format |
| .yg0 | Corel Paradox Secondary Index |
| .yg1 | Corel Paradox Secondary Index |
| .yg2 | Corel Paradox Secondary Index |
| .yg3 | Corel Paradox Secondary Index |
| .yg4 | Corel Paradox Secondary Index |
| .ygf | YWorks Binary Graph Format |
| .ygn | YGen |
| .yif | Mungyodance Game Data |
| .yka | Yenka Model Format |
| .ym | AY-3-8910 Audio Format |
| .ymap | MindGenius Computer Aided Thinking Format |
| .ymbk | Xilisoft Mobile Manager Phonebook |
| .ymg | Yahoo! Messenger Format |
| .yml | YAML Document |
| .yml2 | YML Script Format |
| .ymp | YaST Metapackage Format |
| .ync | YEnc Encoded Format |
| .yog | Outerra Recorded Video Format |
| .yookoo | Yookoo Player Playlist Format |
| .yos | HP Photosmart Essential Thumbnail Cache |
| .ypc | WhyPic Picture |
| .ypf | YahELite Configuration |
| .ypr | BYOB Project Format |
| .yps | Yahoo! Instant Messenger Format |
| .yrcbkm | YRC Weblink Bookmarks Archive |
| .yrm | Red Alert 2: Yuris Revenge Multiplayer Map |
| .yrp | YGOPro Replay |
| .yse | Yamaha Studio Manager Session |
| .ysm | Yamaha Studio Manager |
| .ysp | BYOB Sprite Format |
| .yss | YcySoft Screen Saver Producer |
| .ytf | Google Picasa Font Cache |
| .yti | Google Picasa Plugin |
| .ytif | Independence Instrument |
| .ytr | I.R.I.S. OCR Data |
| .yuv | YUV 16Bits Image Format |
| .yw3 | YWriter 3 Project |
| .yw4 | YWriter 4 Project |
| .yw5 | YWriter 5 Project |
| .ywemanifest | Yahoo! Widgets Manifest |
| .ywl | YoWindow Data |
| .ywp | YWriter 2 Project |
| .yz | YAC Compressed Archive Format |
| .yz1 | Yamazaki Zipper Archive |
