Groves, fighting on the undercard of Robert Guerrero's win over Selcuk Aydin, improves to 15-0.
The Londoner was in control throughout but suffered a deep cut above his eye from a clash of heads in the third round.
The injury puts into doubt the British and Commonwealth super-middleweight champion's headline appearance at Wembley Arena on September 14, where he was expected to drop back to the lighter weight to face Kenny Anderson.
Sierra (25-6-1) targeted the cut, but was undone when Groves threw a straight right-left hook combination.
He got up at the end of the count, but a subsequent flurry from Groves caused the referee to step in.