Mariska Hargitay is mourning the loss of her "Law & Order: Special Victims Unit" co-star Ned Eisenberg.

On Wednesday, the actress took to Instagram and paid tribute to her late castmate, who passed away on Sunday at age 65.

"My heart is so full of sadness over the loss of our dear, dear Ned Eisenberg," the 57-year-old wrote. "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 shared. "We love and miss you, sweet Ned."

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Mariska Hargitay

Mariska Hargitay is mourning the loss of Ned Eisenberg. (Photo by Michael Parmelee/NBCU Photo Bank/NBCUniversal via Getty Images via Getty Images)

The star also posted a picture of the actor smiling while wearing a suit.

"It is a sad day in the SVU history he was a really good lawyer," she captioned the snap with a crying emoji.

Eisenberg died at his New York residence, his representatives confirmed to Fox News Digital.

The beloved onscreen and stage star had been battling two forms of cancer prior to this death: cholangiocarcinoma and ocular melanoma. His agents at Nicolosi & Co. and Ned's wife Patricia commended the star for his brave battle.

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Ned Eisenberg

Ned Eisenberg passed away on Sunday. ( John Lamparski/WireImage)

"As Ned would say, he was attacked by two very rare assassins — cholangiocarcinoma and ocular melanoma," their statement read. "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."

The beloved actor's career spanned four decades. He's known for his role of defense attorney Roger Kressler in "Law & Order: Special Victims Unit."

Additionally, Eisenberg played the role of Sally Mendoza in Clint Eastwood's "Million Dollar Baby," which scored big at the Academy Awards in 2005.

Another one of Eisenberg's notable roles was his portrayal of photographer Joe Rosenthal in "Flags of Our Fathers."

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(L-R) Cast members Jonathan Hadary, Katie McClellan, Ned Eisenberg and Marilyn Matarrese attend ‘Rocket To The Moon’ opening night - curtain call at The Theater at St. Clements on February 23, 2015, in New York City. (Rob Kim)

His other film work included acting in "World Trade Center," "Limitless," "Won't Back Down," "Experimenter," "Asher," "The Exterminator," "The Burning," "Moving Violations," "Air America," "Last Man Standing," "Primary Colors," and "A Civil Action."

On the small screen, Eisenberg starred in "The Fanelli Boys," a comedy also starring Christopher Meloni and Joe Pantoliano.

He had recurring roles in "Mare of Eastown," "The Night Of," "Little Voice," "Person of Interest," "The Black Donnellys," and "Law & Order."

The Bronx native also guest-starred in multiple TV shows, including "The Marvelous Mrs. Maisel," "30 Rock," "The Big C," "The Blacklist," "Bull," "Elementary," "White Collar," "The Good Wife," "Madam Secretary," "The Plot Against America," "New Amsterdam," and more.

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Ned Eisenberg (left) starred as Roger Kessler in ‘Law & Order: SVU’. (Will Hart/NBC/NBCU Photo Bank)

Eisenberg co-founded the Naked Angels Theatre Company in 1987. His professional theater career began in Neil Simon's "Brighton Beach Memoirs," and he also appeared on Broadway in "The Green Bird," "Awake and Sing!," and "Golden Boy."

Most recently, he appeared in the revival of "Six Degrees of Separation," directed by Trip Cullman.

He is survived by his wife, actress Patricia Dunnock, and son, Lino Eisenberg. 

Fox News' Melissa Roberto contributed to this report.