We present the first detection of the envelope which surrounds the post-AGB binary source HR 4049. VINCI-VLTI K-band interferometric observations of this source imply the existence of a large structure with a Gaussian angular FWHM $22.4 \pm 1.4$ mas or uniform disk diameter of $34.9 \pm 1.9$ mas. With the Hipparcos parallax of $1.50 \pm 0.64$ mas these values correspond to a physical size of $14.9^{+11.1}_{-4.4}$ AU and $23.3^{+17.3}_{-7.0}$ AU, respectively. Our measurements, covering an azimuth range of ~60°  for the sky-projected baseline, provide information on the geometry of the emitting region and show that there is only a slight variation of the measured angular values along the different directions sampled. Thus, our results are consistent with a spherical geometry of the envelope. However, we cannot completely rule out the existence of an asymmetric envelope (like the circumbinary disk envisaged by some recent models) because of the limited spatial frequency and azimuth range covered by the observations.