Ultrathin vapor chambers have great potential in cooling compact and portable electronics due to their unique advantages including adjustable cooling surface and good temperature uniformity. However, minimizing the thickness of the vapor chambers while maintaining high thermal conductivity could be mutually exclusive. Here, we develop an ultrathin vapor chamber that enables thermal conductivity of more than 10000 W/mK at an overall thickness of only 0.27 mm. Our ultrathin vapor chamber employs the vapor-liquid coplanar arrangement structure that minimizes the vapor flow pressure drop, the superhydrophilic hybrid mesh wicks that strengthen the capillary performance, and superhydrophilic orthogonal microgrooves that absorb the condensed liquid film and smooth the vapor channels. The heat transfer capability and thermal resistance are theoretically modelled to better understand the heat transfer mechanism of the ultrathin vapor chamber. The proposed extremely thin vapor chamber shows good superiority and great impetus in the thermal management of practical compact applications. This extremely ultrathin vapor chamber and the experimental results may guide the new directions for minimized thermal control devices. •A high thermal conductive vapor chamber with a thickness of 0.27 mm is achieved•Vapor-liquid coplanar structure optimizes the vapor flow pressure drop•Hybrid mesh wicks and orthogonal microgrooves facilitate the liquid flow•The theory model may guide for design of minimum thermal control devices•The developed vapor chamber shows great impetus in cooling compact electronics