Niznik died on June 23, and news of her passing was first announced on Friday. No cause of death has been reported.
“This is heartbreaking,” Wolf shared on Twitter.
“She was so kind, and so fun to work with. I’m lucky to have known her. Rest In Peace Stephanie.”
Niznik played Nina Feeney on all four seasons of the WB drama series. Wolf's Jake was Nina's love interest during the show's final two seasons.
Related: Stephanie Niznik, Everwood Actress, Dead at 52
In the show's series finale Nina ultimately chose to be with Treat Williams' Dr. Andy Brown.
Vivien Cardone, who played Andy’s daughter Delia, has also opened up about Niznik’s passing.
“An absolutely beautiful soul gone far too soon,