She told a hustings meeting organised by Edinburgh University Students Association that she opposed the renewal of Britain's nuclear deterrent.
Ms Boyack returned to the Executive front bench as deputy environment minister in January in the mini-reshuffle that followed Malcolm Chisholm's resignation from the cabinet after he voted with the SNP against Trident.
But she told the hustings in the McEwan Hall that she also opposed the multi-billion pound replacement plan.
Ms Boyack said: "We have reduced the number of nuclear warheads in Britain. I think we could go further."
She also said last week's vote at Westminster had shown there was a large group in the Labour Party opposed to Trident replacement.
The deputy minister added: "The debate is not over. There was a vote in Parliament last week - and that will not be the last word on it."