After eight years, she walked away from the group over a dispute involving her work for SodaStream, a Tel Aviv-based company that operates its main plant in the West Bank. Johansson will be seen in a Super Bowl commercial for SodaStream on Sunday.
Oxfam accepted the actress' resignation, saying it believes SodaStream and other businesses operating in West Bank settlements contribute to the "denial of rights of the Palestinian communities" it works to support.
SodaStream's chief executive says they don't want to sacrifice the jobs of 500 Palestinians who work in their factory due to the political causes of activist groups. Johansson calls it a "fundamental difference of opinion."
Johansson, who has starred in films such as "The Avengers" and "We Bought a Zoo," was one of 17 celebrity ambassadors for Oxfam. Others who are continuing in that role include the band Coldplay, pop singer Annie Lennox and U.S. actress Kristin Davis.
The Associated Press contributed to this report.