The Star Wars creator's GWL Living trust purchased 141,573 shares in the beverage giant for $70.60 (£47) earlier this month (Aug13).
The investment will mean his partner Hobson, who has sat on the Starbucks board since 2005, will have to sign a Form 4 with America's Securities and Exchange Commission to protect herself from allegations of insider trading.
Lucas has plenty of money spare to invest after selling his LucasFilm company and the rights to his Star Wars movie franchise to Disney for $4 billion last year (12).
The director wed Hobson in June (13). They welcomed a child, Everest Hobson Lucas, via surrogate earlier this month (Aug13).