In the DSM-5 (published by the American Psychiatric Association in 2013), five diagnoses describe different aspects of parental alienation:
Parent–child relational problem (p. 715)
A new diagnosis in DSM-5: child affected by parental relationship distress (p. 716)
Another new diagnosis in DSM-5: child psychological abuse (p. 719)
Delusional symptoms in partner of individual with delusional disorder (p. 122), formerly called folie à deux
Factitious disorder imposed on another (p. 325), formerly called Münchausen syndrome by proxy
Bernet, W., Wamboldt, M.Z., Narrow, W. (2016). Child Affected by Parental Relationship Distress. Journal of the American Academy of Child & Adolescent Psychiatry 55:571–579.