The record will live in the Apple Wallet, usually used for payment cards, boarding passes, or event tickets.
Officials say it will first be used to remind people to get their second required dose of the vaccine.
But it could also be used as proof of vaccination in the Covid-19 hotspot of Los Angeles.
Bloomberg first reported news of the county's partnership with software firm Healthvana.
The digital card could be used "to prove to airlines, to prove to schools, to prove to whoever needs it," that a recipient has been immunised, the company's chief executive Ramin Bastani told the newspaper.
Los Angeles County, the most populated county in the United States, has a high level of infection of the disease, with nearly 13,000 new cases reported on Tuesday, and 227 deaths - bringing the number of those who have died with the disease to 9,782 to date.