Barco, 19, arrives on England’s south coast after a promising start to his career in Argentina in which he helped Boca reach last year’s Copa Libertadores final.
He represented Argentina at the U20 World Cup last June and has since won two U23 international caps.
“We have been aware of Valentin for a long time and have been impressed with the progress he has made at domestic and international level,” Brighton technical director David Weir said.
“He is an exciting prospect, but it’s also important we give him the time as he needs to settle and adapt to new surroundings and the Premier League.
“We’re delighted to welcome him to the club, and we are looking forward to seeing how he progresses and develops under Roberto [de Zerbi].”
Barco is away representing Argentina at the South American Olympic qualifiers in Venezuela, which conclude on Feb. 11.
Brighton are eighth in the Premier League with 31 points from 20 matches, and host Wolves on Monday.
Information from Reuters contributed to this report.