How do I tell my therapist I'm in a polyamorous relationship?
Tell them in plain terms early in the work, and pay close attention to how they respond. Affirming clinicians take the structure as given and proceed; non-affirming responses (treating it as the presenting problem when it isn't, or as pathology) signal you may want to find a new clinician.
The simplest approach: mention it in the first session or early second session, when describing your life. Something like 'I'm in a polyamorous relationship — I have a primary partner and one other partner I've been seeing for about a year' is enough. Don't lead with it as a confession; lead with it as ordinary biographical information, because that is what it is.
What you're listening for in the therapist's response. Affirming clinicians will register the information neutrally and ask the kinds of normal clarifying questions they would about any relationship — how long, how it's going, who are these people, what role do they play in your life. They won't ring an alarm bell, they won't move polyamory into the centre of the work uninvited, and they won't treat the structure itself as the diagnosis. The framing will be: this is a relationship configuration, presumably consensual, presumably real, like any other relationship configuration.
Non-affirming responses to look out for: pathologising the structure (treating it as a symptom of avoidance, fear of intimacy, or commitment-phobia, when the structure is presented as a chosen way of being); pivoting the work toward 'understanding why you need this'; visible discomfort or moralising; lack of vocabulary (a clinician who doesn't know what metamour means is fine; one who is openly unwilling to learn the basic vocabulary is a sign).
If the therapist isn't affirming, you have options. You can stay if the rest of the work is good enough and the structure isn't central to what you're working on. You can ask the therapist directly whether they're willing to learn — many clinicians who lack ENM-specific training are responsive when asked, and Kink Aware Professionals, APA Division 44, and several ENM-affirming therapy directories will help find someone with specific competence. You can leave and find a better fit, which is often the right answer.
There are now ENM-affirming clinician directories in most US metros (the National Coalition for Sexual Freedom's Kink and Polyamory Aware Professionals list is one starting point; many community-curated lists exist for individual cities). Asking a local poly community for referrals is also a good path; word of mouth is reliable here.