The treatment uses a virus to deliver a healthy OTOF gene into the ear, which is key for making the protein needed to hear.
Out of 42 participants (from babies to adults), about 90% saw big improvements (many even could start to learn to speak after getting their hearing back).
While early signs show it is safe, doctors say they will keep monitoring everyone.
This breakthrough could open up future treatments for other kinds of genetic or age-related hearing loss too.