Frank McNab (or MacNab) (died 1878) was a member of the Regulators who fought on behalf of John Tunstall during the Lincoln County War. Of Scottish origin, McNab was a "cattle detective" who worked for Hunter, Evans, & Company, which was managed by New Mexico cattleman John Chisum. McNab's job was to track down those who stole Chisum's cattle. Drifting into Lincoln County from the Texas Panhandle in the mid-1870s, he soon signed on with John Tunstall, as his rivals, a group of cattlemen and cowboys from the Seven Rivers area of Lincoln County were allied with Lawrence Murphy and James Dolan.