Simone Giertz, a popular YouTuber with nearly 1 million subscribers who specializes in building robots, said in a video Monday that she has been diagnosed with a substantial brain tumor that will require surgery.

In the video, she said she has had a swollen right eyelid for about a year. After feeling pain in her eye recently, she had an Mri scan last week — which revealed a tumor about the size of a golf ball. “I don’t even like golf, but I do like my brain, a lot,” she said in the video, with her trademark deadpan humor.

Giertz said she will need to undergo extensive brain surgery, which is scheduled for the end of May. “The good news is that I’m probably not dying,” she said. According to Giertz, the tumor appears benign but said that her full prognosis won’t be known until after her operation. She...