This is a YMYL money question, so Mio stays careful: below is general information read from the Japanese-language official sources, not personalised advice. Rules change and your situation is specific — always confirm eligibility with the brokerage or plan provider before acting.
NISA (a government tax-free investment account, run under the FSA) lets you invest in funds/stocks where the gains are tax-free up to annual and lifetime limits — flexible, you can sell and withdraw any time.
iDeCo (individual defined-contribution pension) is a private retirement account where your contributions are deducted from taxable income — but the money is locked until age 60 (in principle). One is for flexible investing; the other is a tax-advantaged pension you can't touch early.
The headline: eligibility is about residency, not passport. For NISA, the common requirements are being a tax resident of Japan aged 18+, with a Japanese address (as printed on your residence card) and a My Number. For iDeCo, per the official iDeCo site you can generally enrol from age 20 up to 65 if you are enrolled in the National Pension and paying (not exempted) premiums — foreign nationals with a valid residence card qualify on the same basis as anyone else.
Clearing the legal eligibility rule doesn't mean any broker will open the account. Japanese brokerages set their own onboarding requirements, and several in practice expect Japanese-language ability or won't process foreign-resident applications smoothly. Some fully support non-Japanese residents; others don't. This is the step where "eligible in theory" and "able to open it today" diverge — so shop around and confirm the broker accepts non-Japanese residents before you count on it.
| Account | When you lose Japanese residency |
|---|---|
| NISA | Once you file your moving-out notice and become a non-resident, new tax-free investing is generally blocked, and you typically must close/handle the account (sell or transfer holdings per your broker's rules). You can usually get your money — but the tax-free wrapper ends. |
| iDeCo | Contributions generally stop, and the funds are locked until age 60 in principle. You may be able to keep the assets invested as an "instruction-only" member, or — if you meet the requirements — claim a lump-sum withdrawal. Do not treat iDeCo money as accessible before 60. |
Because iDeCo money is hard to reach if you leave early, it suits people confident they'll stay long-term. NISA is more forgiving but still loses its tax benefit once you're a non-resident.
If you are a US citizen or US green-card holder, this is often a hard wall. Most Japanese brokers won't open NISA or iDeCo for US persons because of FATCA reporting obligations, and even if you could invest, US PFIC tax rules make most Japanese mutual funds punishing to hold and report. The practical result: many US persons in Japan can't use these accounts at all. If this is you, talk to a cross-border tax professional — this is exactly where generic advice goes wrong.
Long-stay resident, not a US person → NISA and often iDeCo are genuinely worth investigating (with a broker that accepts non-Japanese residents). Short stay → NISA maybe; iDeCo is a poor fit because the money locks until 60. US citizen/green-card holder → assume it's mostly closed to you and get cross-border tax advice. In every case: verify your own eligibility with the provider, because the rule that matters is their onboarding rule, not a general summary. General information, not investment advice. See also the leaving-Japan pension refund guide →