Lemaitre, who opted not to run the 100 at the London Olympics in order to concentrate on the 200, edged out former world champion Kim Collins who ran 10.09.
France's European silver medallist Jimmy Vicaut was shown a red card for a false start.
"Before the race I felt pressure," Lemaitre said. "I am sad that Jimmy Vicaut was disqualified because I feel that I would have run faster with him in the race.
"I would have liked to run faster but I equalled my SB set in Rome at the end of May."
Jamaican Aleen Bailey beat European champion Ivet Lalova of Bulgaria in the women's 100.
New Zealand’s double Olympic champion Valerie Adams won the women's shot put.
"It was an amazing competition," Adams said. "The circle was fast and I enjoyed the atmosphere from the crowd.
"Rieti is an amazing town and I am really looking forward to coming back next year."
Russian high jumper Svetlana Shkolina, who won bronze in London, again beat Olympic and world champion Anna Chicherova just two days after triumphing over her compatriot in the Brussels Diamond League.
Double 5000 and 10,000m world champion Vivian Cheruiyot, who won the silver in the 5000 and the bronze in the 10,000m in London won the women’s 3000m in a final sprint with double world silver medallist Sylvia Kibet.
England's world silver medallist Hannah England was third in the women's 1500m behind Mary Kuria and Anna Pierce.
Former world champion Anita Wlodarczyk, the Olympic silver medallist in the women's hammer, won that competition.
- Jimmy Vicaut