The company is targeting sales of 10% of the £220m dairy shots market within the next few years.
"The category is flat, with sales up 1% and volume down 2%, so retailers have been looking to liven things up," said consumer drinks chief Michelle Frost. "They are excited by the launch as everything on the market is yoghurty and fruit-flavoured."
This was the first chocolate flavour shot, she added. "All the drinks out there are focused on the functional benefits, but with our new product you can get probiotics as well as Galaxy chocolate in 68 calories," she said, adding that Mars' debut would attract new people to the category.
It would also open up usage occasions beyond breakfast, said Frost. "It will attract those who aren't that bothered by probiotics but like the idea of a guilt-free bit of chocolate."
If the Galaxy shot launch was successful, Mars would launch shot variants of some of its other brands, said Frost. It already has milk drinks under its Mars, Maltesers, Milky Way and Bounty brands.
"If this takes off it could revolutionise the market," she said. "Once we have established this category, within one or two years we will look at other brands. If we have rip-roaring sales then we will pull out all the stops and move quickly."
The launch, which has been in the pipeline for two years, will be supported by a TV campaign breaking in mid-November on breakfast TV, as well as in-store activity and a £2 trial price.
The advert's strapline will be 'Real Galaxy chocolate in a probiotic shot. What a delicious idea'. It would focus on the indulgence of Galaxy rather than "the typical bug message", Frost added.
The six-pack (rsp: £2.89) will be rolled out into Tesco, Morrisons and Waitrose on 11 October, with a roll-out at Sainsbury's to follow later.