The deal sees Caffè Nero become the majority owner of the rival group after buying a 70% stake from S.A Brain, which retains a 30% share.
Coffee#1 opened its first location in Cardiff in 2001 and now has 92 sites across Wales, the Midlands and Southern England.
It will remain its own brand and continue to operate and grow as a standalone business under its new owners.
Caffè Nero says the deal is part of its ongoing growth strategy, which sees the c.1,750-site business open a new store worldwide every four days.
John Rhys, chairman of S.A. Brain, says: “The [Coffee#1] Board has resolved that now is the time to join-up with a major player in the coffee market to take it forward to the next stage of its development.
“As ongoing significant shareholders in the business, we look forward to seeing Coffee#1 continue to grow and prosper within the Caffè Nero family over the coming years.”
Caffè Nero has a history of buying out smaller, burgeoning coffee brands. The company acquired the then 43-strong Harris & Hoole chain from Tesco for an undisclosed sum in 2016.
It also bought 26 Aroma coffee sites from McDonald’s for around £3.5m in 2002.