We present Spitzer Space Telescope infrared photometry of a secondary eclipse of the hot Neptune GJ 436 b .
The observations were obtained using the 8- \mu m band of the InfraRed Array Camera ( IRAC ) .
The data spanning the predicted time of secondary eclipse show a clear flux decrement with the expected shape and duration .
The observed eclipse depth of 0.58 mmag allows us to estimate a blackbody brightness temperature of T _ { p } = 717 \pm 35 K at 8 \mu m .
We compare this infrared flux measurement to a model of the planetary thermal emission , and show that this model reproduces properly the observed flux decrement .
The timing of the secondary eclipse confirms the non-zero orbital eccentricity of the planet , while also increasing its precision ( e = 0.14 \pm 0.01 ) .
Additional new spectroscopic and photometric observations allow us to estimate the rotational period of the star and to assess the potential presence of another planet .