In this work, we quantify the mechanical properties of extra-cellular matrix (ECM) in live zebrafish using Brillouin microscopy. Optimization of the imaging conditions and parameters, combined with careful spectral analysis, allows us to resolve the thin ECM and distinguish its Brillouin frequency shift from the surrounding tissue. High-resolution mechanical mapping further enables to directly measure the thickness of the ECM label-free and in-vivo. We find the ECM to be ~400nm thick, in excellent agreement with electron microscopy quantification. Our results open the door for future studies that aim to investigate the role of ECM mechanics for zebrafish morphogenesis and axis elongation.