To transfer United MileagePlus miles, you must log into your account on United.com and navigate to the "Buy, Gift, or Transfer Miles" section. You will need your spouse's MileagePlus number and email address to initiate the transaction. However, transferring miles is rarely the most cost-effective option; as of 2026, the fee is typically $7.50 per 500 miles, plus a $30 processing fee. For example, transferring 10,000 miles would cost approximately $180, which often exceeds the actual value of the miles. A much smarter alternative is to simply book the award flight for your spouse directly from your own account. United allows you to use your miles to book a ticket in anyone's name at no extra charge, bypassing the transfer fees entirely while still using your balance to cover their travel.