Birgen improved his times on the circuit significantly in 2012. He won the Weltklasse in Karlsruhe indoor meet and his time of 3:34.65 was the second fastest indoors that year. This, alongside runner-up placings at the XL Galan and Birmingham Indoor Grand Prix earned him selection for the 2012 IAAF World Indoor Championships, but on the global stage he was eliminated in the first round, the fastest runner not to progress. He knocked over three seconds off his 1500 m best at the Doha Diamond League meet, coming third behind Silas Kiplagat and Asbel Kiprop. A fourth-place finish in the mile run at the Prefontaine Classic saw him set another personal best of 3:50.43 minutes. He did not make the Kenyan Olympic team, coming fifth at the trials, but continued to compete on the 2012 IAAF Diamond League circuit. A run of 3:31.00 minutes ranked him seventh in the world that year, but only fifth in the quick race at Herculis. He was runner-up at the ISTAF Berlin and third at the Van Damme Memorial final.