The 19-year-old is expected to become a future England international regular.
“He’s a young man but he’ll fit into our squad fantastically well,” said Sunderland boss Steve Bruce.
Bruce insists Welbeck is not a straight replacement for Kenwyne Jones, who left Sunderland for Stoke earlier this week.
“It would be unfair to say that he is the replacement,” he said.
“There’s still a couple of weeks to go in the market and hopefully the money is there to go and spend it.”