Enjoying a breakout season in the Bundesliga in 2025/26, Backhaus kept five clean sheets in 32 Bundesliga games to help Die Grün-Weißen preserve their top-flight status.
The 22-year-old has also established himself at international level in recent years, having played for every Germany youth side from the U15s to the U21s.
"[Backhaus] impressively demonstrated the high level of play he can achieve in his first Bundesliga season. His calm and focused manner is contagious," said Freiburg chief sporting officer Jochen Saier.
"He covers all facets of goalkeeping, possesses great potential for development, and will also fit in very well with our team on a personal level. With Mio, we will continue to have a fantastic goalkeeping trio in the future."
