Santos confirmed the move on Monday, with the club also sharing footage of Coutinho arriving at the dressing room and being greeted by Neymar.
"If it's a dream, I don't want to wake up," Neymar told Coutinho in the video.
The reunion is a significant moment for two players who have shared the Brazil shirt but never played together for the same club.
Coutinho spent five years at Liverpool before moving to Barcelona in January 2018. He won two La Liga titles during his time at the Spanish club, although his spell at Barcelona did not overlap with Neymar's, as the forward had already left for Paris Saint-Germain in August 2017.
The 34-year-old Coutinho returned to his boyhood club Vasco da Gama in 2024 but left in February this year. He has not played for Brazil since March 2022.
Neymar returned to Santos last year after his time in Europe and Saudi Arabia. He later announced his retirement from international football following Brazil's World Cup exit in July.
