Marston's said it would take over Brains' 156 pubs in Wales in a deal that would save 1,300 jobs, as Britain's latest round of Covid-19 lockdowns hammer pub and restaurant operators during the crucial Christmas season.
The financial terms of the transaction were not disclosed, but Marston's said the deal would address Brains' funding requirements.
The London-listed company has itself launched consultations on up to 2,150 job losses to deal with the sales hit from lockdowns, and earlier this month called for more government support to bail out the sector.
"As a consequence of the challenges of Covid, and in particular the recently announced more stringent additional trading restrictions in Wales, Brains' business has been under significant financial pressure," Marston's said.
Shares in the pub group rose 2.3% to 70.5 pence in early trade as the company said the deal was expected to add to its earnings in the first year after completion.
The transaction will also raise costs by up to £2m, the company said.
Marston's is a two-century-old brewer known for its Pedigree, Hobgoblin and Lancaster Bomber beers.
It said the Brains' portfolio was complementary with its estate and that it intends to operate the pubs under the Brains' brand.