Are ETFs open-ended funds?

Many ETFs are structured as open-end investment companies (and some as unit investment trusts). What makes ETFs distinct is the creation/redemption process with authorized participants, where large blocks of shares can be created or redeemed, often in-kind.

This mechanism is one reason ETF market prices tend to stay relatively close to the value of the underlying holdings (NAV), though premiums/discounts can still happen.

So while there are exceptions and variations, the “typical” ETF behaves more like an open-end structure than a closed-end fund.