Both are nine digit taxpayer identification numbers, both appear on tax returns, and they are not interchangeable. The distinction is eligibility: the Social Security Administration issues SSNs to citizens and to noncitizens authorized to work in the United States. The IRS issues ITINs to people with US tax obligations who cannot get an SSN. One person is only ever supposed to hold one of them at a time.
What each number can do
An SSN is the broader credential: employment, Social Security benefits, and tax filing all run through it. An ITIN is a tax processing number: filing returns, being claimed on returns, and satisfying institutions that require a taxpayer identification number, such as banks that open accounts for ITIN holders or lenders that offer ITIN mortgage products. An ITIN does not authorize employment and does not create Social Security benefit credit, no matter how many years of taxes are filed with it.
Why someone uses an ITIN instead of an SSN
Because they cannot get an SSN and the tax law still applies to them. US tax obligations follow income and residency tests, not immigration categories, so a person can owe US taxes without being eligible to work. The ITIN exists so those people can comply: file returns, report income, claim spouses and dependents where the rules allow, and build a filing history. A consistent filing history has value far beyond taxes, and immigration and lending processes routinely ask for exactly that record.
When an ITIN holder becomes SSN eligible
This transition happens constantly and has a right way and a wrong way. Once you receive an SSN, stop using the ITIN entirely and notify the IRS in writing so your prior filing history under the ITIN is combined into your SSN record. Do not file under both, and do not abandon the old history: the earnings and filing record you built under the ITIN should follow you. We handle this consolidation for clients making the transition, because a merged, clean record matters for credits, refunds, and everything downstream.