Letang was down on the ice for several minutes midway through the period after he was hit in the head area on a hard check along the end boards by Dallas' Eric Nystrom.
Letang, Pittsburgh's top-scoring defenseman with 31 points, skated shakily to the locker room while Nystrom went to the penalty box to serve a roughing minor.
The Penguins announced in the second period that Letang was done for the night.
"I know (Nystrom) traveled a distance to make the hit," Penguins coach Dan Bylsma said after the game.
Left winger Steve Sullivan also missed all of the third period with the lower-body injury.
Bylsma said both Letang and Sullivan are day-to-day.
Information from The Associated Press contributed to this report.