Malcolm left his home on Cranbourne Road to go for a bike ride about 1:30pm on 12 August.
He usually rides his black mountain bike near the Cranbourne Road – Peninsula Link bike path but has not been seen since.
Police and family hold serious concerns for his welfare due to a medical condition and he has now missed his regular medication.
The 71-year-old is described as Caucasian in appearance, 160cm tall with a medium build and short grey hair.
He was wearing a blue helmet, a black long sleeve t-shirt and grey track pants.
Police on off-road motorbikes, the Air Wing, Mounted Branch and local uniform members are today being supported by the State Emergency Services in searching for Malcolm in Langwarrin and surrounding reserve areas.
Anyone who sees Malcolm is urged to contact Carrum Downs Police Station on 8770 4100.