The 59-year-old Jefferies, a former manager and player at the Edinburgh club, takes over a team who are fifth in the standings but just one point above the relegation group.
Jefferies was in charge when Hearts won the Scottish Cup in 1998 before leaving for Bradford City in 2000. He then moved to Kilmarnock but left the SPL club this month.
"It is not often a manager can come back and it has been nine years. Hopefully, we can use the experience gained in that time. Although I am a bit older the passion is still there for this club," Jefferies told Hearts' website.
The Scot may get a shot at winning some silverware quite quickly with Hearts taking on St Mirren in a Scottish League Cup semi-final on Tuesday. The winners play Rangers or St Johnstone.
Romanian-born Hungarian Laszlo, Hearts' longest-serving manager since Romanov assumed full control in early 2005, achieved a third-place finish in his first season but narrowly missed a Europa League berth in a qualifying match last August.
Romanov has now appointed seven managers - including George Burley, Graham Rix, Valdas Ivanauskas, Anatoly Korobochka, Stephen Frail - during his nearly five years at the club.
