Ernst, 23, and Feyenoord had been in talks for longer and reportedly reached an agreement earlier in the week after it became clear that Feyenoord captain Timon Wellenreuther would leave the club for Wolfsburg.
Tjark Ernst has signed a 4-year contract in Rotterdam.
"It’s wonderful to be able to call myself a Feyenoord player now," Ernst said on the club website. "It felt right from the very first talks. The club has a lot of ambition, and that suits me.
"After three years in the 2. Bundesliga, I feel ready for a move like this. It’s my first time outside Germany, to a big club that plays in European competitions every year and will be back in the Champions League next season."
The 23-year-old German joined Hertha BSC from VfL Bochum in 2023 and has played 93 games for the second-tier side.
Ernst will take up the starting spot at Feyenoord ahead of Belgian-Irish Liam Bossin, while the club may look further for a back-up goalkeeper.
Ernst continued his opening interview by looking forward to Feyenoord's open day, when new arrivals are traditionally unveiled by entering the stadium in a helicopter.
"I can’t wait to take my place between the posts at De Kuip. I know the European matches in particular are experienced with great intensity; Feyenoord is renowned for this not just in the Netherlands. I’m really looking forward to making such an entrance at De Kuip by helicopter on Saturday and meeting the supporters for the first time. It can’t start soon enough for me."
Feyenoord, who fired manager Robin van Persie after the 2025-26 season and replaced him with former manager Giovanni van Bronckhorst, are directly qualified for the league phase of the UEFA Champions League and open their Eredivisie season on Sunday, August 9th, against Sparta Rotterdam.
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