Composite supernova remnants (SNRs) constitute a small subclass of remnants of massive stellar explosions where non-thermal radiation is observed from both the expanding shell-like shock front and from a pulsar wind nebula (PWN) located inside of the SNR. These systems represent a unique evolutionary phase of SNRs where observations in the radio, X-ray and gamma-ray regimes allow the study of the co-evolution of both of these energetic phenomena. In this article, we report results from observations of the shell-type SNR G15.4+0.1 performed with the High Energy Stereoscopic System (H.E.S.S.) and XMM-Newton. A compact TeV gamma-ray source, HESSJ1818-154, located in the center and contained within the shell of G15.4+0.1 is detected by H.E.S.S. featuring a spectrum best represented by a power-law model with a spectral index of $-2.3 \pm 0.3_{stat} \pm 0.2_{sys}$ and an integral flux of F$(>0.42\,\mathrm{TeV}$)=($0.9 \pm 0.3_{\mathrm{stat}} \pm 0.2_{\mathrm{sys}}) \times 10^{-12}$ cm$^{-2}$ s$^{-1}$. Furthermore, a recent observation with XMM-Newton reveals extended X-ray emission strongly peaked in the center of G15.4+0.1. The X-ray source shows indications for an energy-dependent morphology featuring a compact core at energies above 4 keV and more extended emission that fills the entire region within the SNR at lower energies. Together, the X-ray and VHE gamma-ray emission provide strong evidence for the existence of a PWN located inside the shell of G15.4+0.1 and this SNR can therefore be classified as a composite based on these observations. The radio, X-ray and gamma-ray emission from the PWN is compatible with a one-zone leptonic model which requires a low average magnetic field inside the emission region. An unambiguous counterpart to the putative pulsar, thought to power the PWN, has not been detected neither in radio nor in X-ray observations of G15.4+0.1.Read less <