Mary Lockhart said a Yes vote is the best opportunity for socialists to help develop Scotland.
Described as Labour's sister party, the Co-operative has five MSPs including Scottish Labour leader Johann Lamont.
Ms Lockhart said she changed her mind about independence in part because Labour "joined forces" with Conservatives and Liberal Democrats in the Better Together campaign to keep Scotland in the UK.
"I remembered the trades union legislation which Margaret Thatcher introduced, and which Labour failed to repeal, which keeps workers divided," she wrote.
"I pondered a Labour party which had failed to highlight the bedroom tax at earlier stages of the Welfare Reform Bill, a Labour government which had pledged to renew a redundant nuclear deterrent."
Describing her new view on the constitution, she wrote: "It won't deliver Utopia. But it will deliver the chance for socialists to help shape a Scotland which reflects the identity of its people."
Her views were published on the last day of the Labour conference in Inverness.
Jim Lee, secretary of the Scottish Co-operative Party, said its policy position was to support Scotland's place in the UK.
"She is not speaking for us," he said. "We believe in the union."
A spokesman for pro-independence group Yes Scotland said: "More and more labour voters are saying they will vote Yes as they realise that Westminster isn't working for them or their families."