Fitbit says that its devices don't support the Galaxy S9 and Galaxy S9 Plus, a confirmation that it made on Twitter in response to a user asking about the app crashing when trying to open it up and start a run. The response came from Fitbit Support, and although it does note that Samsung's two newest flagship smartphones aren't compatible with its wearables yet there is a silver lining to it all - support should be coming. According to Fitbit support the devices aren't supported just yet but the team is actively working to bring support to the two phones, there's just no time frame at the moment, and technically no guarantee that supported will be added, but it would be strange for one of the biggest wearable companies to forego ensuring support on what will probably be two of 2018's biggest phones.
Fitbit says that as of right now the Galaxy S9 and Galaxy S9 Plus haven't been fully tested so there may be issues with linking either phone to a Fitbit watch or tracker. Both phones meet Fitbit's syncing requirements, so users should be able to connect a Fitbit device to either the Galaxy S9 or Galaxy S9 Plus though it's possible that they may encounter trouble syncing things immediately and may have to make a few attempts, and even then there's still a possibility that may experience crashes with the companion app like the user who reported the problem to Fitbit in the first place.
In short, there's no guarantee that the Galaxy S9 or Galaxy S9 Plus will sync to a Fitbit wearable without any trouble and if they do then there may still be other problems that come up. For the time being Fitbit recommends that users keep an eye on the support page that lists devices which actually do have full support, as this will end up displaying when the Galaxy S9 and Galaxy S9 Plus have been added, as Fitbit is working to get support for them out as quickly as it can.