In July of 2017, Samsung announced that it was partnering with PayPal so that Samsung Pay users could pay for things using their PayPal balance. This was exciting news for a lot of people, but it unfortunately never came to light. Nine months later, this functionality appears to finally be rolling out.
As Joe Kester noted on Twitter, an update that's making its way to the Samsung Pay app in the Play Store adds the ability to link your PayPal account to Samsung's mobile payment service. Once you've downloaded the update, choose to add a new payment method for Samsung Pay and then tap the "Add PayPal" button under the "Add payment card" section.
Tapping this will prompt you to enter your PayPal account info, and following this, you should be good to go. Just like with Google Pay, you'll be able to use your PayPal balance/card at any stores that accept Mastercard.
Samsung does note that it may take 1-2 days for the "Add PayPal" option to show up once you've downloaded the new update, so don't fret if you don't see it right away.