Kemei, who is married to Kirwa, dominated the men’s race. He completed the 21.1-kilometer (13.1-mile) course along Rio’s beaches in 1 hour, 4 minutes and 3 seconds. He was runner-up last year.
Brazil’s Franck Caldeira crossed the line 11 seconds behind Kemei and countryman Damiao Ancelmo was third in 1:04:22.
Kirwa pulled away in the final 500 meters (yards) to win her second consecutive Rio half marathon in 1:14:37, 12 seconds ahead of Alene Amare of Ethiopia.
Kirwa won Sao Paulo’s half marathon in 2008.