The US company is believed to have paid £12.5m for the stake in Ocado which is now valued at £312m.
Ocado services 145,000 customers based mainly in London, Kent, Surrey, Hertfordshire and the Midlands.
Meanwhile, press reports suggest that Waitrose is favourite to pick up the biggest package of Safeway stores from Morrisons. It is expected to buy up to 25 stores for between £250m and £300m.
Morrison must sell 52 supermarkets to fulfil Competition Commission conditions on its £3bn takeover of the Safeway chain.