Marshall Kirk McKusick (born January 19, 1954) is an American computer scientist, known for his extensive work on BSD UNIX, from the 1980s to FreeBSD in the present day.

McKusick lives in California with Eric Allman, his partner since graduate school, whom he married in October, 2013.

In the late 1990s, he implemented soft updates, an alternative approach to maintaining disk integrity after a crash or power outage, in FFS, and a revised version of Unix File System (UFS) known as "UFS2".

The magic number used in the UFS2 super block structure reflects McKusick's birth date: #define FS_UFS2_MAGIC 0x19540119 (as found in /usr/include/ufs/ffs/fs.h on FreeBSD systems).

He was also primarily responsible for creating the complementary features of filesystem snapshots and background fsck (file system check and repair), which both integrate closely with soft updates.