Bigica took over when Alessio Dionisi was sacked last weekend after a run of six league games without a win saw Sassuolo slip into relegation danger. Dionisi had been in charge since taking the reins from Roberto De Zerbi in 2021.
Wednesday's 6-1 home defeat by Napoli meant Sassuolo dropped to 18th place and they decided to act quickly, with Bigica returning to his role as youth team manager.
Ballardini has previously managed Cagliari, Palermo, Lazio, Genoa and Bologna in Serie A. His most recent role was at Cremonese where his team suffered relegation to Serie B and he was dismissed in September after a poor start to this season.
On Sunday, Sassuolo visit Hellas Verona, who are 17th, level on 20 points with both Sassuolo and Cagliari in 19th.
