The 20-year-old confirmed her relationship status on American talk show Red Table Talk which she hosts alongside her mum Jada Pinkett-Smith and grandmother Adrienne Banfield-Norrison.
Polyamory is where those involved agree it is fine for everyone to be open to or have more than one romantic partner.
Willow said she couldn't see herself wanting to "go past two partners".
"With polyamory, I feel like the main foundation is the freedom to be able to create a relationship style that works for you and not just stepping into monogamy because that's what everyone around you says is the right thing to do," she said.
"Also, doing research into polyamory," she added, "the main [reason] …why divorces happen is infidelity."
"It was such a great panel, and I agreed with everything she said especially as it's from a diverse perspective."
Leanne runs Poly Philia which calls itself the largest polyamorous education page in the UK.