The 34-year-old will miss the final game of Samoa's World Cup campaign, as well as Newcastle's four opening fixtures in the 2015-16 Aviva Premiership season.
Tuilagi was cited for striking Japan's Harumichi Tatekawa with his knee. He denied the charge of foul play, but it was upheld after World Rugby reviewed the evidence surrounding the incident.
"Tuilagi denied the act of foul play," said the governing body in a statement.
"Having conducted a detailed review of all the available evidence, including all camera angles, medical evidence, evidence from the player and submissions from his legal representatives, the judicial officer Antony Davies upheld the citing on the basis that the player had committed the act of foul play."
Samoa will take on Scotland in their final World Cup match on Saturday.