Inter have 58 points from their 24 games and top the table from city rivals AC Milan, who have played one game fewer.
Sassuolo lie 11th in the table with 29 points from 24 games.
Dimarco had a hand in goals for Yann Bisseck, Marcus Thuram and Manuel Akanji, while Lautaro Martinez and Luis Henrique were also on the scoresheet as Inter overwhelmed their hosts.
Sassuolo might have had a goal inside the opening minute but Ismael Kone’s shot was cleared off the line by Dimarco, who was effective at both ends of the pitch.
The home side finished with 10 men when Nemanja Matic received a second yellow card for dissent in the 54th minute.
It was a scintillating display from Dimarco, who also struck the crossbar with an audacious free kick from a tight angle.
“He (Dimarco) was one of the best, right now he dominates games. The team is important, but so is the individual level,” Martinez told DAZN.
“We have lost points in important games, we still have to sort out some details to get the results on our side.
“We have to work with our heads down, we are in a great moment and we are happy with the season (so far),” Martinez said.
Dimarco’s corner was headed into the net by the towering Bisseck after 11 minutes, before he put in an inch-perfect low cross from the left wing that was turned into the net by Thuram.
Sassuolo thought they had a goal back before halftime when Domenico Berardi’s attempted shot was mishit but fell kindly for Kristian Thorstvedt to steer the ball into the net, only for the effort to be ruled out for offside.
Inter scored their third goal inside five minutes of the second period as a long throw was flicked on into the danger area, and Martinez drilled the ball low into the net.
It was 4-0 three minutes later as Akanji headed in from Dimarco’s corner, and following Sassuolo protests, Matic was sent off for a second booking, both his cards coming within a few seconds of each other for dissent.
Inter added a fifth when Henrique thrashed home a shot from the edge of the box, his first goal for the club since his move from Olympique de Marseille at the start of the season.