  We present the first multi-wavelength, high-contrast imaging study confirming
the protoplanet embedded in the disk around the Herbig Ae/Be star HD100546. The
object is detected at $L'$ ($\sim 3.8\,\mu m$) and $M'$ ($\sim 4.8\,\mu m$),
but not at $K_s$ ($\sim 2.1\,\mu m$), and the emission consists of a point
source component surrounded by spatially resolved emission. For the point
source component we derive apparent magnitudes of $L'=13.92\pm0.10$ mag,
$M'=13.33\pm0.16$ mag, and $K_s>15.43\pm0.11$ mag (3$\sigma$ limit), and a
separation and position angle of $(0.457\pm0.014)"$ and $(8.4\pm1.4)^\circ$,
and $(0.472\pm0.014)"$ and $(9.2\pm1.4)^\circ$ in $L'$ and $M'$, respectively.
We demonstrate that the object is co-moving with HD100546 and can reject any
(sub-)stellar fore-/background object. Fitting a single temperature blackbody
to the observed fluxes of the point source component yields an effective
temperature of $T_{eff}=932^{+193}_{-202}$ K and a radius for the emitting area
of $R=6.9^{+2.7}_{-2.9}$ R$_{\rm Jupiter}$. The best-fit luminosity is
$L=(2.3^{+0.6}_{-0.4})\cdot 10^{-4}\,L_{\rm Sun}$. We quantitatively compare
our findings with predictions from evolutionary and atmospheric models for
young, gas giant planets, discuss the possible existence of a warm,
circumplanetary disk, and note that the de-projected physical separation from
the host star of $(53\pm2)$ au poses a challenge standard planet formation
theories. Considering the suspected existence of an additional planet orbiting
at $\sim$13--14 au, HD100546 appears to be an unprecedented laboratory to study
the formation of multiple gas giant planets empirically.
