A cyclist out for a Saturday morning ride has been stabbed as he rode past a man and woman in Melbourne's south.

The cyclist had noticed the pair sitting on a footpath and as he rode past, the man stood up and struck him in the chest.

The 32-year-old rode on, not realising he'd been stabbed, before a short time later noticing that he was bleeding from the chest, police said.

The man was riding his bike along Skye Road in Frankston about 10am (AEDT) and was stabbed near McMahons Road.

The victim, from Frankston North, was taken to The Alfred hospital with serious injuries.

Police are searching for the offender, described as Caucasian, aged in his 20s, with brown hair. He was wearing shorts but no shirt.

The woman is aged about 16 or 17, Caucasian and wearing a black outfit.

AAP