"Rest in Peace, Nika," she wrote, adding a red heart emoji and the hashtag "F--- Cancer."
Fans and other stars, including Game of Thrones actor Pilou Asbaek, chimed in on the post.
"F---cancer...Big time!!" Asbeak wrote.
"I'm so sorry for your loss," another person added.
"Condolences," a third comment read. "RIP Nika."
McGuigan, known for her roles in Philomena, Hollyoaks: The Morning After the Night Before, Traders, Mammal and Can't Cope, Won't Cope, is the daughter of Barry McGuigan — a boxer who won several international featherweight titles. He also competed in the 1980 Olympics. The family issued a statement about Nika's death to the public.
"It is with great sadness and heavy hearts that we share the news of the passing of our beautiful daughter and sister, Danika 'Nika' McGuigan," the statement read. "After a brief but brave battle against cancer, Nika passed away peacefully in the early hours of Tuesday 23rd July, surrounded by her loving family. As a family we are devastated and ask for complete privacy during this difficult period to allow us to grief for our Nika."
Nika's brother, famed boxing coach Shane McGuigan, posted his own tribute to the actress on Instagram. He touted how "bravely" she battled cancer until her last breath.
"It breaks my heart to write this. My beautiful sister Danika lost her battle with cancer in the early hours of Tuesday morning, at the young age of 33," Shane wrote alongside photos of Nika. "She overcame it once with a 2 year fight when she was 11 to 13. Myself, my brothers & parents were privileged to have another 20 years wince the first battle with our amazing Nika. Unfortunately Nika couldn't beat it again but she fought so bravely until the end. I have memories that I'll cherish for a lifetime & you will live on forever in spirit. I'm so proud of you! Love you forever my older 'little' sister. RIP."
No funeral arrangements have been made public at this time. It's unclear if her service will be open to fans or not. Her death was only just announced to fans, many of whom are still reeling.