Souayah takes charge of the bid to qualify for next year's African Nations Cup finals, a campaign that is already faltering with the team having picked up only four points from three matches.
The new coach, appointed after a meeting of the Tunisia Football Federation, previously took the north African nation to the 2002 World Cup finals in Japan and South Korea.
Predecessor Benzarti agreed at the start of January to take the job for the second time in just over a year but reversed his decision in the wake of the revolt that last month toppled Zine al-Abidine Ben Ali, the country's president.
Benzarti, who was also Tunisia's caretaker coach at the 2010 Nations Cup in Angola, said the task of qualifying for next year's tournament was too hard and that the contract terms did not suit him.
In an interview with Radio France International he said he was pressurised to take the post by Ben Ali.
"The presidency encouraged me to take the job saying I had to be nationalist and do my best for our people," added Benzarti.
"But now I feel free like the rest of the Tunisian people and we can make decisions not only for the common good but for ourselves too."
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