The young man was attacked on the cycle path at the rear of Cobham Street in Gibshill, Greenock.
He then managed to make his way to Lady Octavia Sports Centre, where a member of the public called the Scottish Ambulance Service.
He was taken by ambulance to Inverclyde Royal Hospital where he is described as being in a serious but stable condition.
“I am appealing to anyone who was on the cycle path or the surrounding area around this time who may have witnessed the attack or saw any suspicious activity to contact the police as a matter of urgency. I am also keen to speak to the person who called the ambulance for the victim.
“We continue to use all available resources at our disposal to investigate this serious incident, and would urge anyone who may have information in connection with the attack, no matter how small they think it is, to contact the police.”