We report on the detection of a transit of the super-Earth 55 Cnc e with warm Spitzer in IRAC’s 4.5 μm band. Our MCMC analysis includes an extensive modeling of the systematic effects affecting warm Spitzer photometry, and yields a transit depth of 410 ± 63 ppm, which translates to a planetary radius of \hbox{$2.08^{+0.16}_{-0.17}\: R_\oplus$}2.08-0.17+0.16R⊕ as measured in IRAC 4.5 μm channel. A planetary mass of \hbox{$7.81_{-0.53}^{+0.58}\: M_\oplus$}7.81-0.53+0.58M⊕ is derived from an extensive set of radial-velocity data, yielding a mean planetary density of \hbox{$4.78_{-1.20}^{+1.31}$}4.78-1.20+1.31 g cm-3. Thanks to the brightness of its host star (V = 6, K = 4), 55 Cnc e is a unique target for the thorough characterization of a super-Earth orbiting around a solar-type star.