Al Hasawi, a member of one of Kuwait's richest families and who resigned as president of Kuwaiti club Qadsia earlier this year, had been linked with the sleeping English giant earlier in the summer.
He had made no secret that he was aiming to buy a second-tier English club, with Forest having only just retained their Championship status at the end of last season.
They had been on the market since the sudden death of owner Nigel Doughty in February.
Al Hassawi confirmed that in a tweet on Tuesday
"I can officially announce that the acquisition of nottingham forest is complete."
"We have become the official owners. I thank my brothers Abdulaziz and Omar for their efforts," he added without providing any financial details on the value of the deal.
The Hassawis have been negotiating the takeover for several weeks with the Estate of Doughty.
In a June 29 statement, the Hassawis said the "future on-field and off-field success of Nottingham Forest is at the heart of our plans."
News of the deal was greeted with delight by the playing staff.
"Great news," said striker Dexter Blackstock in a tweet of his own.
Source: AFP
Source: AFP