The iconic 1980s action hero Sylvester Stallone is normally assumed to just be a Catholic, but it turns out that Stallone’s mother Jackie Stallone (nee Labofish) was born to a jewish father – John Labofish (formerly Labofisz) - and a French mother named Jeanne Clerec. (1)
So thus – despite the website ‘Jew or Not Jew’s’ claim that he is ‘barely a jew’ – (2) in Israeli law Sylvester Stallone is jewish, (3) but he is not jewish according to jewish religious law or halakhah, because his maternal grandfather – not his maternal grandmother – is jewish. Thus, because jewishness is inherited via the mother generally speaking in Judaism; it means that Stallone isn’t jewish according to Judaism, but yet is jewish according to Israel’s official definition.
So yes, Sylvester Stallone is jewish.
References
(1) http://jewishbusinessnews.com/2014/08/18/sylvester-stallones-jewish-mom-forced-him-to-become-a-hairdresser/ ; https://interfaithfamily.com/arts_and_entertainment/popular_culture/Interfaith_Celebrities_Stallones_Jewish_Grandfather/
(2) http://www.jewornotjew.com/profile.jsp?ID=264
(3) https://en.wikipedia.org/wiki/Law_of_Return#Jewish_Ancestry_Amendment