KM3NeT is a multi-purpose neutrino observatory being installed in a phased scheme in the Mediterranean Sea. It is composed of two Cherenkov detectors instrumenting water with photomultipliers in different layouts: ORCA, a compact and dense detector optimised for the measurement of fundamental atmospheric neutrino physics, such as mass ordering and oscillations, in the 1-100 GeV energy range, with unprecedented statistics; and ARCA,
a set of two telescopes with a volume exceeding a cubic kilometer to catch astrophysical neutrinos from 100 GeV to 10 PeV, with a pointing resolution reaching down to 0.1 degree. The two detectors have a final configuration comprising 115 and 230 detection lines, respectively, and currently 18 lines of ORCA and 28 of ARCA are recording data. An overview of first KM3NeT results and prospects will be presented, with focus on the measurement of oscillation parameters, on the search for sources of extraterrestrial neutrinos, and on the prompt multi-messenger program including the search for correlations of neutrinos with gravitational waves. The physics case of KM3NeT is broad and also covers new physics searches that will also be presented, such as non-standard oscillations, invisible neutrino decay and dark matter.