Imagine Dragons' Dan Reynolds Sings 'Whatever It Takes' to Teen Cancer Patient

"Imagine Dragons" frontman Dan Reynolds will do whatever it takes to show love to people who are battling cancer ... even if that means an impromptu performance of one of his band's biggest hits.

Dan was at a comedy show over the weekend in Hollywood with his girlfriend, Minka Kelly, when he serenaded 16-year-old cancer survivor Zoe Suder with a special tune ... and TMZ has it all on video.

Zoe's mom, Leslie, tells us her family was celebrating Zoe finishing 8 months of chemotherapy for stage 4 metastatic bone cancer -- they rented out a lounge area Friday night at the Bourbon Room on Hollywood Blvd.

During his set, comedian Anthony Jeselnik shouted out Zoe, and when Dan caught wind of their celebration ... he went over to her group and sang "Whatever It Takes."

As you can see, Dan shocked Zoe when he came over, said he was super proud of her ... and shared how he's lost friends and family to cancer.

Dan helped launch the Tyler Robinson Foundation after the Imagine Dragons super-fan passed away at 16 from a rare form of cancer. The foundation helps families who have children fighting cancer.

Zoe's got a pretty inspiring story ... she was diagnosed when she was 15, but she's beating the disease and fighting hard to get back to playing soccer, following her dream to one day play in college.