Dejen improved his 3000 m best to 7:34.14 minutes at the XL Galan, but was a little slower at the 2012 IAAF World Indoor Championships, placing fifth in the event. He won the Carlsbad 5000 for a second year running in March 2012. Two 5000 m wins on the 2012 IAAF Diamond League circuit, including a personal best of 12:46.81 minutes, making him the fifth fastest ever over the distance and earning him selection for the 2012 Summer Olympics. At the Olympics in London, he was the fastest qualifier in winning his heat. In the final he chased Mo Farah over the final lap, but did not managed to catch the Briton and took the silver medal behind him. He ended the year with a fourth-place finish at the Weltklasse Zurich, finishing second in the Diamond League rankings after Isaiah Kiplangat Koech.