The brand is targeting commuters during the morning rush hour at Tottenham Court Road, Farringdon, Canary Wharf, Westminster and Green Park.
The stations feature areas complete with branded art work, and ambassadors are offering up bottles of Actimel alongside a leaflet with a 50% off coupon inside.
The £225,000 activity – created by ad agency MEC – kicked off on Monday (11 April) with a performance from the Stay Strong Brothers at Tottenham Court Road station.
Actimel has created the band as part of its wider #StayStrong campaign, which aims to brighten up the daily grind with whimsical songs that encourage positive thinking.
Clips of the five-piece are also being shared across Actimel’s social media accounts.
Cynthia Finke, marketing manager at Actimel, said: "We are really proud to be working with TfL on our new Stay Strong campaign. This is a big part of our global relaunch and we hope it will really resonate with Londoners and bring a smile to their face during their morning commute."
Graeme Craig, director of commercial development at TfL, added: "Our transport network offers a unique opportunity for brands to engage with one of the largest and most diverse audiences in the world.
"By partnering with leading brands like Actimel, we deliver not only substantial revenues to reinvest in transport, but also a more engaging environment for our millions of customers."