Eaton, 27, scored 9,045 points to break his own world record and win gold at the World Championships in Beijing.
Dibaba, 24, set a new 1500m world record of three minutes 50.07 seconds and also winning 1500m gold and 5,000m bronze at the Worlds in August.
The annual awards dinner in Monaco was cancelled because of the corruption and doping issues affecting the sport.
Eaton won the award ahead of Jamaican sprint superstar Usain Bolt, who won three gold medals in Beijing and American triple jump world champion Christian Taylor.
"I did not beat Usain and Christian; my work simply differed in design," said Eaton.
"They are some of the most talented and beautiful performers of all time. I'm flattered to be among them."
Dibaba the sister of three-time Olympic champion Tirunesh, said: "After narrowly missing out on this award one year ago, I am very proud to be recognised by the fans and experts of our sport."
Lord Coe, president of the International Association of Athletics Federations (IAAF), added: "While the athletics family is not gathering together as usual in Monaco, we rightly celebrate the marvellous 2015 achievements of the athletes.
"I offer congratulations to Ashton and Genzebe. Your performances in 2015 are an inspiration and examples of true sporting excellence."
American Ashton Eaton and Genzebe Dibaba of Ethiopia won the IAAF's athlete of the year awards for 2015.