Sanchez, who left Serie A team Inter Milan on Monday by mutual consent, completed his transfer to the French team on Wednesday after passing a medical exam.
Before being released by Inter, the former Manchester United star had been linked with a move to the Ligue 1 team.
Marseille portrayed the 33-year-old forward as a superhero in a cartoon promoting Sanchez’s arrival.
Sanchez joined the 19-time Italian champions in 2019 on an initial loan from Manchester United and scored 20 goals in 109 games.
The former Arsenal striker helped Inter win their first Italian championship in 11 years in the 2020–21 season with seven goals, but he has not been a regular starter since then.
Sanchez, who has 48 goals for his nation and has won the Copa America twice, scored twice in the 2014 World Cup before completing his transfer from Barcelona to Arsenal a month later.
Sunday saw Marseille defeat Reims 4-1 at home to start their Ligue 1 campaign.