GRB 060505 was the first well-known nearby (at redshift 0.089) “hybrid” gamma-ray burst that has a duration longer than 2 seconds but without the association of a supernova down to very stringent limits1. The prompt γ−ray flash lasting ∼ 4 sec could consist of an intrinsic short burst and its tail emission2 , but the sizable temporal lag (∼ 0.35 sec) as well as the environment properties led to the widely-accepted classification of a long duration gamma-ray burst originated from the collapse of a massive star3–5. Here for the first time we report the convincing evidence for a thermal-like optical radiation component in the spectral energy distribution of the early afterglow emission. In comparison to AT2017gfo, the thermal radiation is ∼ 2 times brighter and the temperature is comparable at similar epochs. The optical decline is much quicker than that in X-rays, which is also at odds with the fireball afterglow model6 but quite natural for the presence of a blue kilonova7, 8. Our finding points towards a neutron star merger origin of the hybrid GRB 060505 and strongly supports the theoretical speculation that some binary neutron stars can merge ultra-quickly (within ∼ 1 Myr) after their formation9 when the surrounding region is still highly star-forming and the metallicity remains low. Gravitational wave and electromagnetic jointed observations are expected to confirm such scenarios in the near future.