It is a return to the country where he won the African Nations Cup in 1994 and where he worked at the start of his coaching career with Esperance.
The club said in a statement on Wednesday the 72-year-old would stay until the end of the season in May.
Lemerre was the assistant to Aime Jacquet when France won the 1998 World Cup, stepping up to manage the team after the tournament.
But when France flopped at the 2002 World Cup in Asia, he was dismissed and departed soon after for Tunisia.
Lemerre also worked for a year as national coach of Morocco but was fired after their failure to qualify for the 2010 World Cup.
Etoile are sixth in the Tunisian league standings after eight matches of the season, six points off the leaders.