A total of 6,000 additional rail seats have been installed in the second tier of the Stretford End since September.
That work is now complete and the section has now been officially certified by the Sports Grounds Safety Authority for safe standing.
The new section adds to existing safe standing areas in all four quadrants of Old Trafford and brings the total number of rail seats to 13,577, or 18 per cent of total capacity.
Safe standing is recognised to improve fan safety while also enhancing atmosphere in areas of the ground where fans prefer to stand.