The 21-year-old model from Sherman Oaks was awarded the crown by outgoing Miss USA 2010 Rima Fakih at the pageant at the Planet Hollywood Resort & Casino in Las Vegas on Sunday.
Campanella, who had been asked for her views on legalising marijuana during the question-and-answer segment, will now represent the US at the Miss Universe competition.
Miss Tennessee Ashley Durham - who is best friends with Miss California - was named first runner-up and will assume Miss USA duties if Campanella triumphs at Miss Universe or is otherwise unable to perform her duties.
Miss Alabama Madeline Mitchell was placed third, and Miss Texas Ana Christina Rodriguez finished fourth. Miss Kentucky Kia Hampton won Miss Congeniality USA, while Miss Arizona Brittany Brannon won Miss Photogenic USA.
Judges for Miss USA 2011 included Dallas Mavericks centre Tyson Chandler, chef Rocco DiSpirito, actress and writer Mariel Hemingway, Penn & Teller star Penn Jillett, rapper Lil Jon, Real Housewives of New Jersey regular Caroline Manzo and OPI co-founder Suzi Weiss-Fischmann.
Bravo's Watch What Happens: Live host Andy Cohen and E! News anchor Giuliana Rancic hosted the event, which broadcast on NBC.
Tinie Tempah performed 'Written In The Stars' alongside Eric Turner during the evening gown segment of the pageant.
Miss Universe takes place in Sao Paulo, Brazil on September 12. Miss Mexico Jimena Navarrete won the 2010 competition.
Watch Alyssa Campanella's crowning moment below:
Watch the top four answering their questions in the Q&A round: