Tzimas has penned a contract to 2030 with Brighton, but will spend the rest of the season on-loan with Nurnberg.
The striker joined Nuremburg from PAOK Salonika in the summer on loan, in a move that included an option to buy. Nurnberg activated that option in this current window, ahead of his move to Albion.
Brighton boss Fabian Hurzeler said, "Stefanos is an exceptional young talent, an out-and-out centre-forward with a natural ability to score goals, and we are absolutely delighted to have signed him.
"He will join up with us in the summer, ahead of next season, but we are really looking forward to working with him, and feel he has a very exciting future ahead."
Technical director David Weir also told the club's website, "He will spend the second half of the season with Nurnberg, who have provided him with an excellent opportunity this season, to continue his development in the Bundesliga 2.
"We will keep a close eye on his progress, and wish him the very best for the second half of the season with Nurnberg."