An undercarriage for a rotary wing aircraft, includes a tubular structure (7) with skids, the tubular structure (7) being designed to be mounted under or in a bottom portion (4) of the aircraft, the undercarriage being characterized in that the tubular structure (7) includes left and right longitudinal portions that are retractable, each of which portions is suitable for moving between a retracted position in a cruising flight configuration and a deployed configuration in a landing configuration.