The sealing has involved pumping in specially fromulated mud, forcing oil back down into the well.
BP said the well is being monitored, and further pumping of mud could be necessary.
The company could eventually seal the mouth of the well with cement.
The US government says the well leaked 4.9m barrels of oil before being capped last month, with only 800,000 barrels being captured.
An an explosion on a drilling rig in April which killed 11 workers sparked the oil spill crisis which has taken a severe financial toll on the company.