Republic of Ireland cap Sammon, 30, joined Hearts in June and has scored twice in 25 appearances for the Tynecastle side.
Meanwhile, Scott Boyd, 30, has joined Kilmarnock from Ross County on an 18-month deal after an initial loan spell.
And striker Joshua Umerah, 19, has also moved to Rugby Park on loan until the end of the season from Charlton.
Sammon previously played with Killie between 2008 and 2011 and has had spells with five English clubs.
Defender Boyd has made 13 appearances for Killie since his summer move, scoring once.
Umerah has played three times for the Addicks.
Kilmarnock have signed former player Conor Sammon on loan from Hearts until the end of the season.