  We present the first observational evidence for a circumplanetary disk around
the protoplanet PDS~70~b, based on a new spectrum in the $K$ band acquired with
VLT/SINFONI. We tested three hypotheses to explain the spectrum: Atmospheric
emission from the planet with either (1) a single value of extinction or (2)
variable extinction, and (3) a combined atmospheric and circumplanetary disk
model. Goodness-of-fit indicators favour the third option, suggesting
circumplanetary material contributing excess thermal emission --- most
prominent at $\lambda \gtrsim 2.3 \mu$m. Inferred accretion rates ($\sim
10^{-7.8}$--$10^{-7.3} M_J$ yr$^{-1}$) are compatible with observational
constraints based on the H$\alpha$ and Br$\gamma$ lines. For the planet, we
derive an effective temperature of 1500--1600 K, surface gravity $\log(g)\sim
4.0$, radius $\sim 1.6 R_J$, mass $\sim 10 M_J$, and possible thick clouds.
Models with variable extinction lead to slightly worse fits. However, the
amplitude ($\Delta A_V \gtrsim 3$mag) and timescale of variation
($\lesssim$~years) required for the extinction would also suggest
circumplanetary material.
