If your spouse is physically or mentally unable to apply for Social Security disability benefits on their own, you MAY be able to file on their behalf. However the Social Security Administration (SSA) has rules in order to do so.
According to SSA's Program Operations Manual System (POMS), one must be considered a "proper applicant". A proper applicant is someone who can sign off on an application for benefits on behalf of the disabled claimant. Usually the claimant is considered the proper applicant in most cases as the claimant is the one applying for disability. However, if the claimant is unable to file his or her own application, another proper applicant may be the claimant's spouse, a court-appointed representative, or a relative responsible for the claimant's care.
Physical Inability
If a physical limitation prevents a claimant from signing the application, a spouse or eligible third party may sign it. This could be in situations where the claimant suffered severe burns to the hands, amputations of both arms or hands, coma or even blindness. Keep in mind though, that the Social Security office may require a written statement from the 3rd party applicant identifying the nature of their relationship with the claimant.
Mental Incapacity
If a person has mental limitations such as a severe mental impairment or maybe they've been legally deemed incompetent, a third-party proper applicant may apply on the claimant's behalf. In cases where the applicant is alleging legal incompetence, the Social Security office requires the court order and proof of legal guardianship. A power of attorney is not in itself sufficient. For other claims of mental incapacity, a medical opinion from a treating doctor is usually necessary. If the doctor indicates that the claimant cannot understand the application process, Social Security will send the claimant a letter stating that a claim was filed on their behalf, and allow 10 days for the claimant to object.