The Conservative Party will promise to scrap the Human Rights Act in its election manifesto and announced at its party conference that it could go further by pulling the UK out of the ECHR.
The Scottish Government opposes such a move.
Speaking at a session of Holyrood’s European and External Relations Committee, Ms Hyslop said the decision would have not only practical implications, but would also send a wider message about what the UK stands for.
“From the UK’s position, it has been marginalised in so many different ways in terms of its attitude towards Europe,” she said.
“To remove or to walk away from human rights and the ECHR, and its application within this country, would send out a signal that would further marginalise the UK, not just in Europe but further afield.
“From the UK’s perspective, their influence in terms of empire or the economy or indeed, in many ways, military might, has been diminished over decades, but one of the areas where the UK has a reputation is in terms of fairness and justice and the rule of law and human rights.
“It completely and utterly diminishes any moral authority that they have if they walk away from this.”
Ms Hyslop said the Scottish Government was pursuing discussions with Westminster over what the implications of withdrawal would be for Scotland.
“The idea that the UK would be the first country in the world to go backwards in relation to human rights, I think is fairly incomprehensible,” she said.