On January 2, Heechul’s agency Label SJ and Momo’s agency JYP Entertainment confirmed with outlet EDaily that after having a senior-junior relationship, they recently began dating. - Soompi

According to Soompi, dating rumors between the two began in August 2019, but were initially denied by both parties.

Below is the official statement from Label SJ.

Hello, this is Label SJ. We have confirmed with Heechul and Momo that they have recently started dating from being a close sunbae and hoobae. Thank you. 

Below is the official statement from JYP Entertainment.

Hello, this is JYP Entertainment. We are confirming that Heechul and Momo are in a relationship. They have recently started a relationship that grew from a sunbae-hoobae friendship in the industry. Thank you.