The body has joined the UK Council for Psychotherapy and the British Psychological Society in saying that using the practise to encourage someone to identify as a different gender (male, female, non-binary or gender fluid) is also not supported by any evidence.
In a new memorandum of understanding, they stated: "Signatory organisations agree that the practice of conversion therapy, whether in relation to sexual orientation or gender identity, is unethical and potentially harmful.
"Signatory organisations agree that neither sexual orientation nor gender identity in themselves are indicators of a mental disorder."
The document, which is also backed by the Royal College of General Practitioners and the National Counselling Society, updates a previous memorandum issued two years ago which focused exclusively on sexual orientation.
It defines conversion therapy as any therapeutic approach assumes one gender identity is "inherently preferable to any other, and attempts to bring about a change of sexual orientation or gender identity, or seeks to supress an individual's expression of sexual orientation or gender identity on that basis."
The organisations stated that studies into the use of conversion therapy should be kept under regular review, and that healthcare professionals and psychological therapists are aware of the "ethical issues" relating to conversion therapy.
They added: "This position is not intended to deny, discourage or exclude those with uncertain feelings around sexuality or gender identity from seeking qualified and appropriate help.
"This document supports therapists to provide appropriately informed and ethical practice when working with a client who wishes to explore, experiences conflict with or is in distress regarding, their sexual orientation or gender identity."