The 18-year-old will sign a five-year contract on July 1st, subject to international clearance.
"Charalampos is an outstanding young player and we're delighted he is coming to Brighton," Seagulls boss Fabian Hurzeler said.
"He wants to play in the Premier League, and we are excited about what he can bring to the team.
"He will give us different attacking options, and we're looking forward to helping him adapt to his new environment."
Kostoulas was part of the Olympiacos side that completed a league and cup double last season, scoring seven goals with two assists in 35 appearances in all competitions.
Kostoulas is the third Brighton signing since the end of the season, following the arrivals of Sunderland winger Tom Watson and South Korean teenager Yoon Do-young.