"My heart is so full of sadness over the loss of our dear, dear Ned Eisenberg," Hargitay, 57, wrote in a message. "What a light and what a love. And such a first-rate actor, which pales next to him as a first-rate human."
"We will remember him always with his bright, mischievous smile and his wide-open heart," she added, before concluding, "We love and miss you, sweet Ned."
Hargitay -- who stars as Captain Olivia Benson on "SVU" -- also shared a sweet photo of her late co-star smiling.
She captioned her post, "Once an SVU Alum, always always always an SVU Alum. His time may be Dun Dun, but this was his story…💙👨🏻💼💼 🙏🏼 #DARogerKressler #SVUAlum #LongMayHeReign #SVUniverse #RestinPowerNed."
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Eisenberg played defense attorney Roger Kressler in "Law & Order" and "Law & Order: SVU," the latter of which he appeared in 24 episodes. He was most recently seen in the critically-acclaimed HBO series, "Mare of Easttown." According to his IMDb page, the New York City native had a whopping 77 acting credits -- in both TV and film -- throughout his career, with the first credit being in 1987.
As first reported by TMZ on Sunday, Eisenberg died at the age of 65 after battling two rare cancers: cholangiocarcinoma -- also called bile duct cancer --, as well as ocular melanoma, which is an extremely rare, yet the most common form of eye cancer.
"As Ned would say, he was attacked by two very rare assassins -- cholangiocarcinoma and ocular melanoma," Eisenberg's wife Patricia told TMZ in a statement. "Over the course of two years, he bravely fought the cancers in private while continuing to work in show business to ensure that his medical coverage paid for himself and his family."
In addition to Patricia, Eisenberg is also survived by their son, Lino.