The Atari XF351 was a planned 3.5" floppy disk drive that never saw mass production. It is considered "vaporware" (promised but not released), and only rare prototypes exist officially. The drive was planned for release around 1985–1987. One of the reasons for its cancellation was the reluctance of software developers to support the new media format (3.5") for the old 8-bit platform, while 5.25" disks remained the standard. Instead of the XF351, Atari released the XF551, which supported standard 5.25" disks but supported double density.

Since the original device is extremely rare, enthusiasts have created modern functional replicas. One of them is the replica from Zaxon. This modern device in a 3D-printed case allows the use of 3.5" floppy disks (DD 720KB or HD) 1.44 MB) on original Atari hardware.