He has been ruled out for the three remaining games of the series.
Smith fell headfirst after jumping over the boundary to stop Mahesh Theekshana from scoring six runs in the final over.
He grabbed the ball one-handed, and he flicked it back into play in a stunning display of skill. However, he was able to stay on the ground for a brief moment after landing heavily.
Match officials looked at replays while Smith was being carried off the ground by team medical staff and determined it was a six as Smith’s foot touched the boundary.
Sri Lanka scored 18 runs in last over to tie it, but Australia won in Super Over to win the series 2-0.
Cricket Australia released a short statement Monday morning to confirm Smith’s concussion. They said Smith would be “subjected to low-level protocols” for several more days, with Smith expected to make a full recovery within seven days.
The Australians did not plan to add Smith’s replacement.
The next game will take place Tuesday in Canberra. Sri Lanka needs to win this game to maintain the five-match series.
Smith, 32, is one of the most highly-ranked cricketers in the world. He is part of an Australian test team that will travel to Pakistan later in March for a three-match series. It starts in Rawalpindi on March 4. This will be Australia’s first Pakistan tour since 1998.