Nico Robin rejoins the Straw Hat Pirates in "One Piece" during the "Enies Lobby" arc. More specifically, in episode 299, titled "Fists Full of Emotion! Luffy Unleashes Gatling with All His Might!" Robin openly expresses her wish to live and return to the crew after Luffy and the others fight fiercely to retrieve her from the World Government.
Robin had initially left the crew at Water 7, revealing her affiliation with the CP9, a World Government agency. However, it was revealed later that she was blackmailed into leaving her crewmates to protect them.
The Straw Hat Pirates did not accept this and declared war against the World Government to get her back. In the climactic moments of their battle in Enies Lobby, Robin shouts out that she wants to live and rejoin the Straw Hat Pirates. This marks her official return to the crew.
Learn more: