Around 400 buses in the city will carry ads from Macmillan Cancer Support highlighting the next phase of the charity's Not Alone campaign.
Experts say 18,100 people in Glasgow are living with cancer and that number is expected to increase to around 31,800 by 2030.
The advertising space donated by First will help raise awareness of the support available.
Employees at First Glasgow and its parent company FirstGroup selected Macmillan as their national charity partner in 2012 and have so far raised more than £687,000 in cash and in kind.
Sally Hill, partnership manager at Macmillan Cancer Support, said: "We need to ensure people are aware of the support available to them and First's support of our Not Alone campaign is very important as a result."