Educational Institute of Scotland (EIS) president-elect Nicola Fisher said the policy “borders on eugenics”.
The UK Government policy limits families to claiming tax credits for their first two children.
One of the exemptions is the so-called ‘’rape clause’’, which requires women to prove their third child was conceived through rape or during an abusive relationship to qualify for the benefit.
Ms Fisher moved a motion calling for support for the campaign against the cap and condemning the rape clause on the grounds it will compound poverty and deprivation and further traumatise women who have been subject to sexual violence, which was passed at the EIS National Council.
Scottish Labour leader Kezia Dugdale welcomed the decision to condemn the rape clause.
Ms Fisher said: “Imagine for a moment what it would be like to be child number three.
“To be the child in your family who the state considers less deserving, less worthy of support but more of a burden on your family.
“To be the child that the state considers really should never have been born.
“This policy is nothing short of shameful and borders on eugenics.
“It is certainly an attempt at social engineering.”
Kezia Dugdale said: “I welcome the EIS decision to condemn the Tories’ horrific rape clause.
“The union recognises what all decent people across our country recognise, which is that a policy that forces rape victims to fill out a form to receive vital child benefit for their children has no place in our society.
“Instead of shaming rape victims, we should be shaming Ruth Davidson and the Conservatives for supporting this vile policy. Scots will remember that every single Conservative MSP voted in favour of the rape clause just a few weeks ago. It shows the true nature of the Tories - they don’t stand up for our Union, they simply drive the burden of austerity onto the most disadvantaged members of society.”