Margaret Christian "Peggy" Garnett Jay (4 January 1913 – 21 January 2008) was an English Labour councillor.
As a young girl, Peggy Garnett attended St Paul's Girls' School in London, where she befriended Shiela Grant Duff. In 1931, she went up to Somerville College, Oxford, but she left two years later to marry Douglas Jay. She led a long political career with the British Labour Party and served as a London County Councillor, recruited by Herbert Morrison, representing Hackney then Battersea.
She was the last of the "Hampstead middle-class Labour grandes dames" whom Morrison groomed to take over the London County Council in 1934. She was elected to the new Greater London Council before losing her seat in 1967.
She married politician Douglas Jay in 1933, aged 20. They had four children, but the marriage ended in divorce.
A son, Peter Jay, is a leading economist and a former British Ambassador to the United States and a son-in-law is Rupert Pennant-Rea, a former deputy Governor of the Bank of England.
Jay's niece, Virginia Bottomley, a Conservative, served in the cabinets of Margaret Thatcher and John Major.