Do 401(k)s fall under Box 3, or does that only apply when distributions are taken?

A standard U.S. employer-sponsored 401(k) is generally not taxed in Dutch Box 3 while the money remains inside the plan.

The Dutch tax authorities usually treat a 401(k) as pension rights rather than as ordinary savings or investments. This means the plan balance is normally excluded from Box 3 wealth tax as long as it remains within the pension plan and has not been distributed.

If distributions are later received while the person is a Dutch tax resident, those payments are generally taxed in Box 1 as pension income at the applicable progressive income tax rates, rather than in Box 3.

That said, the exact treatment can depend on the type of account, how the plan is structured, and whether withdrawals have already been made. For that reason, it is always worth checking the details before reporting a U.S. retirement account in the Netherlands.