The 23-year-old edged out France's Jimmy Vicaut and American Ronnie Baker, who were both clocked at 10.05 seconds.
"My focus was to be free and run free," said Ujah, who is now looking forward to the World Athletics Championships in London in August.
"London is the main one. I hope I can do something special then."
Ujah's fellow Briton was sixth in the women's 100m as the Netherlands' Dafne Schippers took victory from Marie-Josee Ta Lou of the Ivory Coast and Michelle-Lee Ahye of Trinidad and Tobago with a time of 10.99.
equalled her season's best of 1.91m for fourth in the women's high jump, won by Russia's world champion Mariya Lasitskene (formerly Kuchina), while the country remains suspended from international athletics.
ran 13.24 secs for fourth in the men's 110m hurdles, won by American Aries Merritt in 13.13, while was seventh in the women's pole vault, won by Greece's Katerina Stefanidi with a world-leading height of 4.85m.
was 16th in the women's 5,000m with a season's best of 15 minutes, 24.05 seconds. The race was won by Kenya's Hellen Obiro in 14:18.37, a world leading time and national record, while did not finish.
Chijindu Ujah was born in Enfield, London, into a family who are originally from Nigeria. He grew up in Enfield and has one older sibling. Alongside his brother he attended Bishop Stopford's School. Chijindu went on to study at Sir George Monoux College, a sixth form college in Walthamstow, where he took up Science and Maths. He is currently a sport science student at Middlesex University in London.