Look up a lost Social Security number by submitting an application along with necessary documentation to a local Social Security office in person or by mail. You cannot replace a lost Social Security number card online or by phone. Alternatively, look up forgotten Social Security numbers on old tax returns.
Use Form SS5 to replace a Social Security number card as well as apply for a new one. Find and download the form on the Social Security Administration website, or obtain one at a Social Security office. Along with the form, bring proof of your age and U.S. citizenship such as a U.S. birth certificate, U.S. passport, Consular Report of Birth Abroad, Certificate of Naturalization or Certificate of Report of Birth. If you are a non-citizen with permission to work from the Department of Homeland Security, bring proof of current immigration status, such as a Lawful Permanent Resident Card, Employment Authorization Document or Certificate of Eligibility for Non-immigrant Student Status. A second document verifying identity should be a U.S. driver's license, state-issued non-driver ID card or a combination of other photo-ID cards.
The required documents must be originals or copies that are certified by the original issuing agency. Notarized copies are unacceptable. The Social Security Administration recommends keeping Social Security number cards in a secure place and not carrying them on your person unless they are needed for identity by employers, banks or government agencies.