Samsung has given us yet another major indication that the Android 6.0 updates for its latest devices could be rolling out soon. The Samsung Pay app has received an update that readies the app for Marshmallow support – Samsung Pay is an important service for the company, and it would make sense for it to be among the first apps that become Android 6.0 compatible. The Smart Switch app was updated recently to include 6.0 support as well.
The Samsung Pay update brings the app's version number to 1.6.6.1, and the changelog notes Marshmallow compatibility and performance and UI enhancements. You should get a notification for the update when you open Samsung Pay on your phone; in case you don't, hit the More button and head into Settings » About Samsung Pay to update manually.