Stewart is Southampton boss Russell Martin’s top attacking target and Saints saw two bids rejected before they were able to agree a deal with Sunderland.
The Black Cats will bank £8m plus a further £2m in add-ons for Stewart.
With the striker currently injured, Southampton have been keen to go through a medical carefully and he has now come through the medical checks.
The medical has been completed and Southampton will now be looking to sign him.
Stewart is out of contract at Sunderland next summer and was not prepared to sign an extension.
And the Black Cats have chosen to cash in on the player as a result.
It remains to be seen when Stewart will be fit enough to return to the pitch, but this time with Southampton.
