[UPDATED] So Ji Sub has also confirmed the news via his agency through an Instagram post:
So Ji Sub is now off the market! Sorry mga bes, may nanalo na!
Early last year, So Ji Sub announced that he is dating announcer, Jo Eun Jung and asked for the love and support of his fans.

So Ji Sub’s agency, 51K, has also released an official statement regarding this matter:
Hello, this is 51K. We have some good news about our actor, So Ji Sub. He met someone and developed a precious relationship with her. The two had been dating for the past 19 months and has promised to be partners for life based on trust and love for each other. So Ji Sub and Jo Eun Jung is registering their marriage today, April 7, 2020.
The couple decided to keep their marriage private and wants to cherish their special day with the important people in their lives.
We would like to thank everyone who supported So Ji Sub and we hope that you continue blessing him and his wife on their new journey together. So Ji Sub will continue his best to work hard as an actor.
Annyeong Oppa wishes So Ji Sub and Cho Eun Jeung a happy and everlasting marriage!
Source: (1)